Support starting PD with separated IP when using playground

Feature Request

Is your feature request related to a problem? Please describe:

Describe the feature you'd like:

Support the following command or something similar:

tiup playground nightly --kv=1 --pd=1 --pd.Host 172.16.4.1 --db=0 --host 172.16.4.2

--pd.Host specifies where PD is located, and --host specifies where TiKV and other binary are located.

Right now, I find we have already had this flag when executing tiup playground --help

tiup is checking updates for component playground ...
A new version of playground is available:
   The latest version:         v1.11.0
   Local installed version:    v1.5.6
   Update current component:   tiup update playground
   Update all components:      tiup update --all

Starting component `playground`: /data2/ryan/.tiup/components/playground/v1.5.6/tiup-playground --help
Bootstrap a TiDB cluster in your local host, the latest release version will be chosen
if you don't specified a version.

Examples:
  $ tiup playground nightly                         # Start a TiDB nightly version local cluster
  $ tiup playground v5.0.1 --db 3 --pd 3 --kv 3     # Start a local cluster with 10 nodes
  $ tiup playground nightly --monitor=false         # Start a local cluster and disable monitor system
  $ tiup playground --pd.config ~/config/pd.toml    # Start a local cluster with specified configuration file
  $ tiup playground --db.binpath /xx/tidb-server    # Start a local cluster with component binary path
  $ tiup playground --mode tikv-slim                # Start a local tikv only cluster (No TiDB or TiFlash Available)
  $ tiup playground --mode tikv-slim --kv 3 --pd 3  # Start a local tikv only cluster with 6 nodes

Usage:
  tiup playground [version] [flags]
  tiup [command]

Available Commands:
  display
  help        Help about any command
  scale-in
  scale-out

Flags:
      --db int                   TiDB instance number
      --db.Host host             Playground TiDB host. If not provided, TiDB will still use host flag as its host
      --db.Port int              Playground TiDB port. If not provided, TiDB will use 4000 as its port
      --db.binpath string        TiDB instance binary path
      --db.config string         TiDB instance configuration file
      --db.timeout int           TiDB max wait time in seconds for starting, 0 means no limit
      --drainer int              Drainer instance number
      --drainer.binpath string   Drainer instance binary path
      --drainer.config string    Drainer instance configuration file
  -h, --help                     help for tiup
      --host string              Playground cluster host
      --kv int                   TiKV instance number
      --kv.binpath string        TiKV instance binary path
      --kv.config string         TiKV instance configuration file
      --mode string              TiUP playground mode: 'tidb', 'tikv-slim' (default "tidb")
      --monitor                  Start prometheus and grafana component
      --pd int                   PD instance number
      --pd.Host host             Playground PD host. If not provided, PD will still use host flag as its host
      --pd.binpath string        PD instance binary path
      --pd.config string         PD instance configuration file
      --pump int                 Pump instance number
      --pump.binpath string      Pump instance binary path
      --pump.config string       Pump instance configuration file
      --ticdc int                TiCDC instance number
      --ticdc.binpath string     TiCDC instance binary path
      --ticdc.config string      TiCDC instance configuration file
      --tiflash int              TiFlash instance number
      --tiflash.binpath string   TiFlash instance binary path
      --tiflash.config string    TiFlash instance configuration file
      --tiflash.timeout int      TiFlash max wait time in seconds for starting, 0 means no limit
  -v, --version                  version for tiup

Use "tiup [command] --help" for more information about a command.

From the help message, it could be misleading since it says "Playground PD host". But I did try it, and the following error happens:

panic: can't get a free port

goroutine 1 [running]:
github.com/pingcap/tiup/pkg/utils.MustGetFreePort(...)
	github.com/pingcap/tiup/pkg/utils/freeport.go:42
github.com/pingcap/tiup/components/playground/instance.NewPDInstance(0x0, 0x0, 0xc0005cef00, 0x23, 0x7ffedb70f104, 0xb, 0x0, 0x0, 0x0, 0xc000226f98)
	github.com/pingcap/tiup/components/playground/instance/pd.go:42 +0x1e5
main.(*Playground).addInstance(0xc000350f00, 0x556dac99e1e6, 0x2, 0x0, 0x0, 0x0, 0x0, 0x1, 0x7ffedb70f104, 0xb, ...)
	github.com/pingcap/tiup/components/playground/playground.go:617 +0x3f7
main.(*Playground).bootCluster(0xc000350f00, 0x556dad1c3908, 0xc0004c6940, 0xc000230890, 0x556dad7ead00, 0x0, 0xc000230890)
	github.com/pingcap/tiup/components/playground/playground.go:723 +0x86b
main.execute.func2(0xc0004d2a00, 0xc0000fe0c0, 0x1, 0xc, 0x0, 0x0)
	github.com/pingcap/tiup/components/playground/main.go:238 +0x4a5
github.com/spf13/cobra.(*Command).execute(0xc0004d2a00, 0xc0001229d0, 0xc, 0xc, 0xc0004d2a00, 0xc0001229d0)
	github.com/spf13/cobra@v1.1.3/command.go:852 +0x472
github.com/spf13/cobra.(*Command).ExecuteC(0xc0004d2a00, 0xc000219c70, 0x1, 0x1)
	github.com/spf13/cobra@v1.1.3/command.go:960 +0x375
github.com/spf13/cobra.(*Command).Execute(...)
	github.com/spf13/cobra@v1.1.3/command.go:897
main.execute(0xc0d2faf54cd85c9d, 0xf6dfe7)
	github.com/pingcap/tiup/components/playground/main.go:300 +0x1445
main.main()
	github.com/pingcap/tiup/components/playground/main.go:569 +0x58

Why the featue is needed:

It could be very useful when we doing tests and we have already supported integrating some testing tools with the playground. See
https://github.com/tikv/pd/pull/5289
https://github.com/tikv/pd/pull/5688

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

Start by reproducing the documented playground command and trace the failure through components/playground/main.go, components/playground/playground.go, and components/playground/instance/pd.go, with free-port handling in pkg/utils/freeport.go. Check how --pd.Host and --host are passed when creating instances. Done means the separated-address command starts without the reported panic and the help text matches the supported behavior.