Use CogStack API for deidentification

[milanmlft]

Replace EHR deidentification with a call to the CogStack API.

• Set up mock cogstack service with FastAPI for tests
• Add PIXL_EHR_COGSTACK_REDACT_URL environment variable to configure cogstack
• Remove old deidentification code

Closes UCLH-Foundry/the-rolling-skeleton#159

To do

• Fix test_message_processing() unit test
• Add documentation
• Remove obsolete deidentification code

[stefpiatek]

Imagine you may be doing it already but we can remove all of the spacy and en_core_web_lg in documentation and from the ehr Dockerfile

[stefpiatek]

Thanks for getting this in Milan 🎉

[stefpiatek]

Tested on GAE05 and its doing what it should. Names have been replaced with [name], new fields for cogstack like HCPC number have not been redacted. Think we're good for a merge if you're happy with my Dockerfile changes