Avoid using go embedded messages in Config

[bogdandrutu]

This PR:

• Helps with [confmap] Embedded structs fail to unmarshal #12709, to avoid the situation where we need to add Unmarshal for a struct and if used as embedded will not work.
• Helps with avoiding name conflicts between variables. Will allow for example to have a "Timeout" member in both QueueConfig and ClientConfig, even though that was a valid YAML config since QueueConfig is under "seding_queue".
• This is a breaking change only if devs are created manually the OTLP configs which should be very rare, and unfortunately this is hard to do using deprecation steps.

[dmitryax]

I guess, we should add this as a guideline in https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/coding-guidelines.md#configuration-structs

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.54%. Comparing base (abcee1f) to head (61893b4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
receiver/otlpreceiver/otlp.go	0.00%	0 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (93.75%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12718   +/-   ##
=======================================
  Coverage   91.54%   91.54%           
=======================================
  Files         480      480           
  Lines       26456    26456           
=======================================
  Hits        24218    24218           
  Misses       1772     1772           
  Partials      466      466           

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