Implement a loggregatorreceiver

[ctlong]

Implement a loggregatorreceiver locally that could receive all input from Forwarder Agent in loggregator v2 format. Rather than converting loggregator v2 to OTLP in Forwarder Agent, we we should convert loggregator v2 to OTLP in the OTel collector, this release.

This has the following benefits:

• Removes "special" code from the Forwarder Agent that is only used by this release.
• Adds functionality to this release that could be extended to eventually allow it to replace Forwarder Agent.

Tasks

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1. Implement loggregatorreceiver
2. Update otel-collector job to register a loggregator v2 endpoint for Forwarder Agent
3. Remove the otelcolclient from Forwarder Agent

[ctlong]

I made this WIP branch with a spike that seemed to work, though I didn't test it thoroughly: https://github.com/cloudfoundry/otel-collector-release/tree/wip/loggregatorreceiver.