[outputs.http] support secret store for client_id and client_secret in

### Use Case

Storing secrets at a secret store instead of in some random location in the filesystem would be great and probably more secure.

### Expected behavior

It would be great if client_id and client_secret could bet properly added through the secret store as well as username/password.

### Actual behavior

currently you need to put the client_id and client_secret in one of the toml config files and make sure to properly restrict the access to these files, especially on Windows.

### Additional info

_No response_

Start with the outputs.http plugin and its current username/password secret-store handling. Trace how client_id and client_secret are loaded from TOML and compare the existing secret-store path. Done means both fields can be supplied through the secret store without requiring TOML values, with tests covering the configuration.

Written by the indexing model from the issue text.