Investigate adding a `Validate()` function to `HTTPClientSettings`

[atoulme]

          I think we should have a PR in core that adds a "Validate" on the `HTTPClientSettings` for this instead of duplicating everywhere.

Originally posted by @bogdandrutu in open-telemetry/opentelemetry-collector-contrib#29931 (comment)

[atoulme]

This is now implemented. Closing.