Support for Custom Exporter Authenticators as Extensions

[pavankrish123]

Is your feature request related to a problem? Please describe.
We recently added framework to add server (receiver) side authentication via extensions. This is very convenient to extend and add custom authentication seamlessly. In similar fashion clients (exporters) should be able to utilize any custom authentication defined as extension.

Describe the solution you'd like

something like this (similar to extension/authoidcextension) for receivers

extensions:
  oauth2:
      client_id: someclientidentifier
      client_secret: someclientsecret
      token_url: https://autz.server/oauth2/default/v1/token
      scopes: ["some.resource.read"]

exporter:
  otlphttp/withauth:
    authentication:
      authenticator: oauth2

service:
  pipelines:
    traces:
      receivers: [otlp]
      processors: []
      exporters: [oltphttp/withoauth]

Describe alternatives you've considered
Implemented PR #2464 where ConfigHttp client directly implements one form of Auth (Oauth2)

[pavankrish123]

cc: @jpkrohling @bogdandrutu

[bogdandrutu]

@jpkrohling would let you drive this, but feels reasonable

[jpkrohling]

Looks good, just not 100% about the property name. We use authenticators on the receivers' side as well, which would make this consistent but might confuse users. Do you think we can solve this with documentation by ensuring that all authenticators clearly state whether they are meant to be used on the server/receiver side vs. client/exporter?

[rakyll]

If this happens, we may consider having SigV4 as an extension.

cc @alolita @Aneurysm9 @anuraaga

[pavankrish123]

This is great!

@jpkrohling I was pondering on the same. The right place to get this information would be in the extension config definition. Not sure what is the best way.

In any case, I think we need to refine the interface here https://github.com/open-telemetry/opentelemetry-collector/blob/main/config/configauth/authenticator.go#L36 to more specific cases. like HTTPServerAuthenticator, GRPCServerAuthenticator, HTTPClientAuthenticator, GRPCClientAuthenticator WDYT.?