Add indexes

[loneil]

#425

Add indexes for the fields pres_exch_id and pyop_auth_code for the auth_session collection.

Those fields are used to look up on at various points in the VCAuthn flow so they should need indexing.
Can look for get_by_pyop_auth_code and get_by_pres_exch_id for usage in the code.

This just uses the pymongo create_index command in the init_db part of the app, which is called on startup of the oidc-controller.
This successfully added indexes on my local where there are around 500 documents already populated.
Note for consideration: do we need to think about index addition for running VCAuthn instances that may(?) have a lot of documents in the collection? Would that need millions? Billions? in order to be concern with an index addition? Not sure if we are close to that in any existing VCAuthn contexts using the auth_session collection.

Result right after adding and restarting controller:

[Jsyro]

Looks good!

[jamshale]

Definitely seems like the correct indexes.

What is the auth_code? This is a unique identifier for the login? Or is this common number code? It won't help and might hurt if a lot of the entries are the same.

[esune]

Definitely seems like the correct indexes.

What is the auth_code? This is a unique identifier for the login? Or is this common number code? It won't help and might hurt if a lot of the entries are the same.

The auth_code is uniquer per authentication attempt, it won't be shared by sessions so it is safe to have an index on it if we are querying that collection based on its value.