enhancement(http-sink): Add automatic bearer token acquisition for http-sink

[KowalczykBartek]

This PR intend to be a proof-of-concept for #20635.
Because I have little knowledge about vector and not much more experience in Rust, I started with minimal working implementation to just verify concept (for example, where code responsible for token acquisition should be placed :)), if shape of this change will be more or less ok, I will take care for test, docs and code quality :)

example configuration I used to test this solution.

sources:
  my_source_id:
    scrape_interval_secs: 1
    type: http_client
    endpoint: http://127.0.0.1:9898/logs
    auth:
      strategy: "o_auth2"
      client_id: "some.client.id"
      client_secret: "some.client.secret"
      token_endpoint: "https://some.oauth.system/oauth/token"

sinks:
  out:
    inputs:
      - "my_source_id"
    type: "console"
    encoding:
      codec: "text" 

[bits-bot]

All committers have signed the CLA.

[tareksha]

i suggest prefix the env var names with OAUTH_ or OAUTH2_