Implement M3 test client about rt-5gms-application-server

[davidjwbbc]

httpx works very well. To do HTTP/1.1 with HTTP/2 upgrade you can use:

client = httpx.AsyncClient(http1=True, http2=True, headers={'Connection': 'upgrade', 'Upgrade': 'h2c'})

...and for HTTP/2 prior knowledge:

client = httpx.AsyncClient(http1=False, http2=True)

I've set the test client to prior knowledge to match the other 3GPP NF interfaces.

[davidjwbbc]

Added the cache purge operation to the test client. This can take two forms:
m3_client_cli.py -H <host>:<port> purge <provisioning-session-id> - This will purge all cache entries for the provisioning session (pattern field in the request body is not present).
m3_client_cli.py -H <host>:<port> purge <provisioning-session-id> <regex> - This will purge all cache entries for the provisioning session where the cached URL path matches the regex (sent in the pattern field of the request body), the URL path used for the comparison is the full path of the object on the backend origin.

[davidjwbbc]

Implemented a basic client which can do certificates list/add/update/delete and content hosting configuration list/add/update/delete. Code needs tidying up and M3 operations separating into a module, away from command line parsing, for re-use in unit tests.

[davidjwbbc]

Code tidied up and pushed to the feature-add-m3-interface branch on my fork.

[davidjwbbc]

Realised the test client is only talking HTTP/1.1, need to switch from the requests module to the httpx module in order to use HTTP/2. Not sure if this module does prior knowledge for HTTP/2 (will test with httpx.AsyncClient(http1=False, http2=True) to see if it'll go straight to HTTP/2).