Support specifying credentials for OTLP exporter

[jfallows]

Describe the desired outcome from the user's perspective
As a system admin, I need to specify the user credentials for my OpenTelemetry collector.

Acceptance criteria

• Provide config options to indicate authentication credentials that only apply to the OpenTelemetry exporter requests

Additional context
See #1369 for equivalent enhancement to schema registry catalog.

[jfallows]

Suggested syntax

  otlp0:
    type: otlp
    options:
      ...
      credentials:
        headers:
          authorization: Bearer <token>