Update proxy.go - Use json format when querying tsh apps

[sonal-ridecell]

Currently if your terminal window is too small or text size is large, the proxy command fails with this error -

ERROR the teleport application with ridectl label does not exists or you do not have access to it, error while executing command: exit status 1

This is because the tsh apps ls command strips the whole output based on available space & hence grep is not able to find ridectl even if you have access. Ex (notice the trailing dots in the end) -

Application                      Description Type Public Address                   Labels                                            
-------------------------------- ----------- ---- -------------------------------- ------------------------------------------------- 
> ibm-labload-intelligence-da...             TCP  ibm-labload-intelligence-data... app_name=ibm-labload-intelligence-data-db-acce... 

Using --format=json removes the restriction of terminal size and prints the whole apps info. Ex -

[
  {
    "kind": "app",
    "version": "v3",
    "metadata": {
      "name": "ibm-labload-intelligence-data-db-access",
      "labels": {
        "app_name": "ibm-labload-intelligence-data-db-access",
        "ridectl": "true",
        "teleport.dev/origin": "config-file"
      }
    },
    "spec": {
      "uri": "[redacted]",
      "public_addr": "[redacted]",
      "insecure_skip_verify": false
    }
  }
]

Have tested the PR by building ridectl locally