Add support for mTLS

[or-ram]

Proposed changes

Describe the use case and detail of the change. If this PR addresses an issue on GitHub, make sure to include a link to that issue using fix keyword here in this description and in corresponding commit message.

Checklist

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• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes

[azlato]

This PR adds mTLS support for OTLP export.

Why it matters:

• It lets nginx-otel authenticate to collectors that require client certs.
• It adds the missing pieces: client key, client cert chain, and test coverage.

I need this change too, and I think it would be very useful to get more eyes on this PR.

For reviewers, the main things to check are:

• config directives: client_private_key and client_certificate_chain
• how cert/key are loaded and passed into gRPC SSL credentials
• whether the new mTLS test covers the intended collector auth flow

This looks small, focused, and practical. Please take a look when you can.