Allow passing a custom reporter to the controller

[dmathieu]

Right now, the Collector receiver implementation (in #160) uses OTLPReceiver, which sends data over the wire to an OTLP server.

This is not the behavior we want for the collector. We need to use nextConsumer to pass the data down to the next stage, and have it respect the collector's config.
To achieve that, we need to be able to pass a custom reporter to the controller, so a CollectorReporter can build pdata from the traces, and pass it down to the next consumer.

[dmathieu]

See also draft #208 (WIP) for the main custom collector we'll need to pass in the collector receiver.

[christos68k]

Not sure if we should have this here as in some cases (e.g. long-running operation that can fail) it's going to create problems (reporter that hasn't initialized properly with the rest of the agent being oblivious and executing as normal).

I'd reword to the following or remove altogether:

Suggested change

	// If the reporter needs to perform a long-running starting operation then it
	// is recommended that Start() returns quickly and the long-running operation
	// is performed in the background.
	// If this method needs to perform a long-running operation that can NOT fail, it is
	// recommended that Start returns quickly and the long-running operation is
	// performed in the background.

[dmathieu]

I don't know. Running an HTTP server can fail, yet that's a long-running operation that should not run synchronously here.
If folks need something long-running that can report errors, they should setup a channel, or an error handler to be able to report the error upstream.

Note that this comment is heavily inspired by the one on collector component, which does the same thing.
https://github.com/open-telemetry/opentelemetry-collector/blob/main/component/component.go#L32