[Do not merge] Agent api prototype

[LikeTheSalad]

Prototype that should cover what's being discussed here.

• A Kotlin factory method that returns OpenTelemetryRumBuilder, which to me it seems to be the simplest way of using the agent as there's only one method that users would need to be aware of. It also contains config params for the default instrumentations.
• Following the diagram, it removes some API surface from OpenTelemetryRumBuilder.
• Following the diagram, It moves the "unserializable" config params from OtelRumConfig into OpenTelemetryRumBuilder.
• Following the diagram (kind of, as the interface initially named SessionManager is now changed to SessionProvider Session handling implementation opentelemetry-js-contrib#2358) it sets the SessionProvider as the only session-related type in core, and moves all of our session-related types to the agent so that the existing session handling tools (the ones that create sessions based on a timeout) are provided by the agent, and "power-users" can provide their own session handling logic directly to core, if needed.
• Following the diagram, it creates EndpointConfig and a default impl that provides HTTP endpoints.
• Adds the OTLP exporters dependency to the agent module and makes the agent to automatically set otlp-http span and log exporters that use the provided endpoint config params.
  Here we can see how this agent API is used in our demo app.