[FEAT] Implement plugable authentication for OTLP HTTP

Many systems expose an OTLP HTTP endpoint.

Currently, the only auth supported by opentelemetry-cpp in the OTLP HTTP exporter, is to provide an http header in plain text, with magic key/value pairs, used to represent a long lived auth token (immutable for the life cycle of the exporter).

opentelemetry-cpp needs to be expanded to:

• Define an OTLP HTTP authentication interface
• Allow the OTLP HTTP exporter to use an arbitrary authentication
• Sign HTTP requests using the authentication implementation provided
• Notify the authentication implementation of failures, to support rotation of credentials

When an application wants to talk to an endpoint using the XYZ authentication method, the application is then responsible to provide an authentication plugin that implements the XYZ method.

Authentication plugins are a natural extension point for file configuration, see related:

• https://github.com/open-telemetry/opentelemetry-configuration/pull/275

Sub tasks:

• Move all the compression code from the HTTP layer to the OTLP layer
• Isolate a compression interface
• Implement compression in a separate code base for gzip
• Isolate an authentication interface
• Implement authentication in a separate code base for basic auth
• Change OTLP HTTP exporters to accept a custom compression
• Change OTLP HTTP exporters to accept a custom authentication
• Support rotation of SSL certificates
• Support rotation of credentials