Spec config to enable/disable OTel bridge

[gregkalapos]

Specify the ELASTIC_APM_OPENTELEMETRY_BRIDGE_ENABLED config in the OTel bridge spec.

This config is not relevant for all agents, but some (I know about Node.js and .NET) have an explicit config to enable/disable the OTel bridge. Goal of the PR is to make sure all agents use the same config name.

Discussion from .NET: https://github.com/elastic/apm-agent-dotnet/pull/1966#discussion_r1060551833. The name is taken from the Node.js implementation. So far the only agent which needs to adapt is .NET that I know of.

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#### Build stats * Start Time: 2023-01-30T05:49:00.034+0000 * Duration: 3 min 9 sec