feat(prometheus_remote_write sink): add custom HTTP headers support

[elohmeier]

Summary

Add support for custom HTTP headers in the prometheus_remote_write sink via the request.headers configuration option. This enables users to add custom headers to outgoing requests for authentication, routing, or other integration requirements with Prometheus-compatible backends.

Vector configuration

[sinks.prometheus_remote_write]
type = "prometheus_remote_write"
endpoint = "https://prometheus.example.com/api/v1/write"

[sinks.prometheus_remote_write.request.headers]
X-Custom-Header = "custom-value"
X-API-Key = "secret-key"
X-Event-Tag = "{{ some_field }}"  # Template support for dynamic values

How did you test this PR?

• Added unit test sends_custom_headers() that verifies custom headers are correctly added to requests
• Tested custom docker build with Thanos Receive using a custom Tenant ID HTTP header

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Please merge #23773 first. I'll then rebase this PR and resolve the conflict.

Notes

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