Calling the GetWatchers() function results in a stacktrace

## What happened?

Calling the GetWatchers() function results in a stacktrace [1]

This occurs because the Watchers api no longer returns an AccountID
for each Watcher and now returns a Key and a Username. The Get User
functions also must be updated to support Key (by default) and Username.

Signed-off-by: Prarit Bhargava

[1] panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x6c0d48]

goroutine 1 [running]:
github.com/andygrunwald/go-jira/v2/onpremise.(*IssueService).GetWatchers(0xc000334030, {0x9c5eb0, 0xc000028120}, {0xc000029c18?, 0xc0002136a0?})
/home/prarit/go/pkg/mod/github.com/andygrunwald/go-jira/v2@v2.0.0-20230325080157-2e11dffbdb9a/onpremise/issue.go:1373 +0x208
gitlab.com/prarit/rhjira/cmd.GetWatches(0xc000242100)
/home/prarit/Other/gitlab/rhjira/cmd/lib.go:72 +0x57
gitlab.com/prarit/rhjira/cmd.glob..func1(0xc6e4e0?, {0xc00019a230?, 0x1?, 0x1?})
/home/prarit/Other/gitlab/rhjira/cmd/dump.go:265 +0x5d2
github.com/spf13/cobra.(*Command).execute(0xc6e4e0, {0xc00019a200, 0x1, 0x1})
/home/prarit/go/pkg/mod/github.com/spf13/cobra@v1.7.0/command.go:944 +0x847
github.com/spf13/cobra.(*Command).ExecuteC(0xc6e200)
/home/prarit/go/pkg/mod/github.com/spf13/cobra@v1.7.0/command.go:1068 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
/home/prarit/go/pkg/mod/github.com/spf13/cobra@v1.7.0/command.go:992
gitlab.com/prarit/rhjira/cmd.Execute(0xc000334030?)
/home/prarit/Other/gitlab/rhjira/cmd/main.go:25 +0x3e
main.main()
/home/prarit/Other/gitlab/rhjira/main.go:34 +0x15b

## What did you expect to happen?

GetWatchers() succeeds and doesn't panic.

## How can we reproduce it (as minimally and precisely as possible)?

I don't know if there is an easy way to reproduce the panic. One thing to try is to simply call GetWatchers() on an issue that has watchers. The returned list will be empty because the Watchers api no longer returns an AccountID for each Watcher and now returns a Key and a Username

## Anything else we need to know?

Calling the GetWatchers() function results in a stacktrace [1]

This occurs because the Watchers api no longer returns an AccountID
for each Watcher and now returns a Key and a Username. The Get User
functions also must be updated to support Key (by default) and Username.

Signed-off-by: Prarit Bhargava

[1] panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x6c0d48]

goroutine 1 [running]:
github.com/andygrunwald/go-jira/v2/onpremise.(*IssueService).GetWatchers(0xc000334030, {0x9c5eb0, 0xc000028120}, {0xc000029c18?, 0xc0002136a0?})
/home/prarit/go/pkg/mod/github.com/andygrunwald/go-jira/v2@v2.0.0-20230325080157-2e11dffbdb9a/onpremise/issue.go:1373 +0x208
gitlab.com/prarit/rhjira/cmd.GetWatches(0xc000242100)
/home/prarit/Other/gitlab/rhjira/cmd/lib.go:72 +0x57
gitlab.com/prarit/rhjira/cmd.glob..func1(0xc6e4e0?, {0xc00019a230?, 0x1?, 0x1?})
/home/prarit/Other/gitlab/rhjira/cmd/dump.go:265 +0x5d2
github.com/spf13/cobra.(*Command).execute(0xc6e4e0, {0xc00019a200, 0x1, 0x1})
/home/prarit/go/pkg/mod/github.com/spf13/cobra@v1.7.0/command.go:944 +0x847
github.com/spf13/cobra.(*Command).ExecuteC(0xc6e200)
/home/prarit/go/pkg/mod/github.com/spf13/cobra@v1.7.0/command.go:1068 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
/home/prarit/go/pkg/mod/github.com/spf13/cobra@v1.7.0/command.go:992
gitlab.com/prarit/rhjira/cmd.Execute(0xc000334030?)
/home/prarit/Other/gitlab/rhjira/cmd/main.go:25 +0x3e
main.main()
/home/prarit/Other/gitlab/rhjira/main.go:34 +0x15b

## Your Environment

Include as many relevant details about the environment you experienced the problem in

* go-jira version (git tag or sha): this is actually seen with 1.16.0 but also can be reproduced with 2.0.0 (top-of-tree commit ID 2e11dffbdb9a4710fdc9233e8efe0a124e204e24)

* Go version (`go version`): go1.19.9
* Jira type (cloud or on-premise): on-premise
* Jira version / Api version: 2