[BearerAuth] Implement Bearer Token Auth for HTTP Clients

[pavankrish123]

Is your feature request related to a problem? Please describe.
Current implementation for Bearer token auth only supports gRPC clients only as the same effect can be achieved by setting "headers" option in exporters with {Authorization: Bearer *****}

Describe the solution you'd like

extensions:
  bearertokenauth:
    token: "somerandomtoken"

receivers:
  hostmetrics:
    scrapers:
      memory:

exporters:
  otlphttp/withauth:
    endpoint: https://someendpoint:8090
    ca_file: /tmp/certs/ca.pem
    auth:
      authenticator: bearertokenauth

Describe alternatives you've considered
We can also make do with directly setting headers but that is not clean nor consistent with grpc clients

Additional context
Add any other context or screenshots about the feature request here.