feat: add support for user_id in header

[cpacker]

Please describe the purpose of this pull request.

Adds basic support for reading user_id from the header of the HTTP request.

Note: implementation is not thread safe and will be / needs to be refactored alongside the stateless server refactor.

How to test.

Test no header:

$ curl --request GET \
     --url http://localhost:8283/v1/models/ \
     --header 'accept: application/json'
[{"model":"gpt-4","model_endpoint_type":"openai","model_endpoint":"https://api.openai.com/v1","model_wrapper":null,"context_window":8192}]%

Test header with bad ID (no user exists):

$ curl --request GET \
     --url http://localhost:8283/v1/models/ \
     --header 'accept: application/json' \
     --header 'user_id: 0x0'
{"detail":"User with id 0x0 not found"}%

Test header with existing ID (the config one):

[client]
anon_clientid = user-6122fbc6-ef57-413e-a84c-7965f42151fe

$  curl --request GET \
     --url http://localhost:8283/v1/models/ \
     --header 'accept: application/json' \
     --header 'user_id: user-6122fbc6-ef57-413e-a84c-7965f42151fe'
[{"model":"gpt-4","model_endpoint_type":"openai","model_endpoint":"https://api.openai.com/v1","model_wrapper":null,"context_window":8192}]%