Implement client key file and client certificate file for all OTLP exporters.

[chittalpatel]

Description

Added support to provide client key file and client certificate file to all OTLP exporters.

Fixes #2991 Fixes #3327

Type of change

• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Updated tests and verified them using: tox -e opentelemetry-exporter-otlp-proto-grpc tox -e opentelemetry-exporter-otlp-proto-http

Does This PR Require a Contrib Repo Change?

Answer the following question based on these examples of changes that would require a Contrib Repo Change:

• [x] No.

Checklist:

• [x] Followed the style guidelines of this project
• [x] Changelogs have been updated
• [x] Unit tests have been added
• [ ] Documentation has been updated

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: srikanthccv / name: Srikanth Chekuri (975b64cc1f6e27f1fbe999821e944ddc00129bd3)
• :white_check_mark: login: xrmx / name: Riccardo Magliocchetti (df4c50258a54fd14e59b25eb02f56777bf4501fc)
• :white_check_mark: login: chittalpatel / name: Chittal Patel (5c7e9a3c7dbef162895b86eef460f953caf69140, 6bef4e640cfb7027f874c2b017784d478ead46ef, 39bc279a1858267e7ec041aab9f47805fc0623b8)

[emike922]

I would be interested in this feature. Are you still working on this?

[frankthetank001]

Also interested in this. Is MTLS not supported on all exporters? I'm running into this problem a few times today with Node JS http exporter also