validate tiflash spec config part

if write it as:

config: |
   TOML-format...

the tc can be apply success
but operator failed to work like

E0705 07:07:22.933371       1 tidb_monitor_controller.go:92] TidbMonitor: testbed-gn9qg/tc, sync failed, err: get tm[testbed-gn9qg/tc]'s target tc[testbed-gn9qg/tc] failed, err: v1alpha1.TidbCluster.Spec: v1alpha1.TidbClusterSpec.TiFlash: v1alpha1.TiFlashSpec.Config: readObjectStart: expect { or n, but found ", error found in #10 byte of ...|"config":"## Log lev|..., bigger context ...|s":[{"topologyKey":"zone"}]},"tiflash":{"config":"## Log levels: trace, debug, info, warning, error,|...

it should be:

config:
  config: |
     TOML-format...
  proxy: |

we should validate the spec first make it failed to be created

there are maybe other components we should check together