feat(sinks): enable sinks with token auth.

[pburrows-ns1]

Enable sinks to be configured with token based authentication.
The required fields for this auth type are:

1. scheme
2. token

Rather than scheme being fixed to Bearer, it's customizable because there are many schemes it could be.
For example Dynatrace expect the header to be:
Authorization: Api-Token dt0c01.abc123.abcdefjhij1234567890

With the following authentication settings configured on a sink:

{
	"exporter": {
		"endpoint": "https://acme.com/otlphttp/push"
	},
	"authentication": {
		"type": "bearertokenauth",
		"scheme": "Api-Token",
		"token": "abcdefg"
	}
}

The resulting yaml configuration is:

---
receivers:
  kafka:
    brokers:
    - kafka:9092
    topic: otlp_metrics-sink-id-22
    protocol_version: 2.0.0
extensions:
  pprof:
    endpoint: 0.0.0.0:1888
  bearertokenauth/withscheme:
    scheme: Api-Token
    token: abcdefg
exporters:
  otlphttp:
    endpoint: https://acme.com/otlphttp/push
    auth:
      authenticator: bearertokenauth/withscheme
service:
  extensions:
  - pprof
  - bearertokenauth/withscheme
  pipelines:
    metrics:
      receivers:
      - kafka
      exporters:
      - otlphttp

This is in line with what's expected:
https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/4115aad52e0775caed38c3ccc450d8064044efaa/extension/bearertokenauthextension

[mfiedorowicz]

Hey @pburrows-ns1, thanks for the first-time contribution/pull request 👏🏻 We'll do our best to review it in a timely manner.

[mfiedorowicz]

Thanks @pburrows-ns1! Apart from a few minor or code style comments and one clarifying question, it looks like a great contribution 👏🏻

[mfiedorowicz]

@pburrows-ns1 could you also merge develop into your branch which has a fix for one of GHA workflows failing here (plus one conflict requiring resolving)? 🙏🏻

[mfiedorowicz]

Looks good @pburrows-ns1, thanks for contribution!