feat(otlp-exporter-base)!: use transport interface in web exporters

[pichlermarc]

Which problem is this PR solving?

This PR applies the pattern from #4743 to the browser exporters. The first commit in this draft is #4889, as existing tests rely on fixing this behavior first.

This PR makes it so that retry logic is shared across both Node.js and Browser, and by implementing the IExporterTransport interface for sendBeacon and XHR, moves us closer to combining all base classes.

With this change, you'll also notice that the code from all OTLPExporterNodeBase, OTLPExporterBrowserBase, and OTLPGRPCExporterBase all start looking very similar to each other, only differing in their constructor. In a follow-up I'll tackle configuration, which will allow us to combine all three into a shared base. See #4415 for how this will look like.

Breaking changes:

• (internal) removes the export for sendWithXhr()
• (user-facing) protected headers property was intended for internal use has been removed from all exporters

Once configuration is individually testable, we can reduce the existing tests for the exporters to higher-level integration tests as opposed to the very deep integration/unit tests that assert the state of the now individually tested sub components.

Part of #4116
Requires #4889

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• Adapted existing tests
• Manual tests using the opentelemetry-web example

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 3 lines in your changes missing coverage. Please review.

Project coverage is 92.16%. Comparing base (ecc88a3) to head (aef7d15).
Report is 71 commits behind head on main.

Files	Patch %	Lines
...xporter-base/src/platform/browser/xhr-transport.ts	93.10%	2 Missing ⚠️
...base/src/platform/browser/send-beacon-transport.ts	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4895      +/-   ##
==========================================
+ Coverage   91.04%   92.16%   +1.11%     
==========================================
  Files          89      158      +69     
  Lines        1954     3471    +1517     
  Branches      416      761     +345     
==========================================
+ Hits         1779     3199    +1420     
- Misses        175      272      +97     

Files	Coverage Δ
...base/src/platform/browser/send-beacon-transport.ts	92.30% <92.30%> (ø)
...xporter-base/src/platform/browser/xhr-transport.ts	93.10% <93.10%> (ø)

... and 67 files with indirect coverage changes