[FEATURE]: Support refreshing provider credentials without restarting cli

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Enable configuring a token refresh command and interval for api keys with custom providers, this is useful when fetching the api key from a vault or IdP.

Currently when fetching an api key from IdP/vault it is done through env vars when launching opencode but they cannot be refreshed without closing opencode, generating a new token and opening opencode back up.

We should provide the ability to specify a script to generate the token and the interval to refresh it.

Eg:

{
  "provider": {
    "my_provider": {
      "auth": {
        "command": "/path/to/get-token.sh",
        "refreshInterval": 3600
      }
    }
  }
}

Similar to claude apiKeyHelper and the codex auth config which enable refresh commands.

I am happy to implement this feature