What does this PR do?

When a RUM Session ID is available, we set it in the Baggage header of intercepted network requests when using trackResources=true.

We don't account for pre-existing values in the Baggage header, so we end up overwriting it.

This PR adds support for user-defined Baggage headers by using comma-separated list strings, following the W3C Baggage specification.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)
• If this PR is auto-generated, please make sure also to manually update the code related to the change