Add otlp_receiver package

[graphaelli]

Add an OpenTelemetry Protocol (OTLP) Receiver package that enables Elastic Agent to function as a managed OpenTelemetry Collector. We already have EDOT Collector released for standalone use and Fully Managed OTLP Ingest on Cloud - this rounds out the options.

Acceptance criteria:

required:

• configurable grpc listen address, default 0.0.0.0:4317
• configurable http listen address, default 0.0.0.0:4318
• toggle log acceptance, default enabled
• toggle metric acceptance, default enabled
• toggle trace acceptance, default enabled
• toggle profile acceptance, default enabled (hidden until profiles are available)
• toggle producing aggregated metrics from traces (elasticapm connector)
• elasticapm processor always enabled for traces when they are accepted
• api key auth for SDKs available, enabled by default
• bearer token auth for SDKs available, disabled by default

Default listen addresses can be all interfaces as the default configuration includes authentication requirements. This will reduce issues for users running under docker too.

Out of scope (probably):

• Gateway mode - ability to configure an OTLP output.
• Additional processor configuration(s) - TBS, transform, etc

[elasticmachine]

Pinging @elastic/ecosystem (Team:Ecosystem)