feat: Dynamic Headers support for OTLPExporters

[bozzelliandrea]

Which problem is this PR solving?

Still working progress to verify all use cases, tested locally with HTTP for Authentication header use case.

This PR add the dynamic header support where you can set a function (sync or async) and every signal sent through the exporter will execute and use the function result as http header value.

Fixes #2903

Usage example: demo-instrumentation

Short description of the changes

• Types now support string and Function for headers
• Header builder execute function param, resolve if needed the promise, and set the value

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• ✅ Created a demo app (linked on the description), deps link to fork with new feature, and tested HTTP exporter with Auth standard header.

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added (TODO)
• Documentation has been updated (TODO)

[linux-foundation-easycla]

• ❌ - login: @bozzelliandrea / name: Andrea Bozzelli . The commit (371b53f) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.