Custom TrafficExtension for Multi-Tenancy

The charm has to support multi-tenancy according to the design outlined in the official spec.

Add the logic to operate the required TrafficExtension for user-ID header enforcement from source namespace.

The logic to operate the required TrafficExtension for user-ID header enforcement from source namespace is added.

Internal ticket manually created, because of a bug with Jira's automation (reason why the label for Jira was removed): https://warthogs.atlassian.net/browse/KF-8906

Testing

green path: from kubeflow profile with an alias -> can create experiment + http call to get user identity returns expected kubeflow profile identity
red path: from kubeflow profile with incorrect user id header (spoofing): http call to get user identity while setting the header with an incorrect identity -> output: actual correct identity not the spoofed one

Closed by #502