feat(new_relic sink): add override_endpoint configuration option

[tracyatrf]

Allow users to configure a custom API endpoint for the New Relic sink, enabling use with other non-production endpoints.

Summary

This PR adds a new override_endpoint configuration option to the new_relic sink. When set, this URL takes precedence over the region and api settings for determining where data is sent. This is useful for New Relic developers or users who need to send data to staging environments or other non-production endpoints.

Vector configuration

[sinks.my_newrelic]
type = "new_relic"
inputs = ["my_source"]
license_key = "${NEW_RELIC_LICENSE_KEY}"
account_id = "12345"
api = "metrics"
override_endpoint = "https://alternative-metric-api.newrelic.com/metric/v1"

How did you test this PR?

• Reviewed existing test coverage which already exercises the override_uri code path
• Updated the existing test to use the new override_endpoint config field instead of the internal override_uri field

[github-actions]

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[tracyatrf]

I have read the CLA Document and I hereby sign the CLA