Testing: Add type hints to utils.py and FilterEngine, refactoring code where needed

[rdimaio]

Part of #6588.

b3227b1

• The BaseClient constructor is refactored in a way where the static type checker can detect that host and auth_host are never None.

d5d3d67

• Type annotations in utils.py. The methods here are used in a few places, and there are some instances where the input types are unclear, so I added type: ignore comments in places where the issue seems to be with the parent function rather than the util function. Some notes below.

oidc.py

- 1 errors with message """Argument of type "str | Unknown | Any" cannot be assigned to parameter "vallist" of type "list[str]" in function "val_to_space_sep_str"

  Type "str | Unknown | Any" cannot be assigned to type "list[str]"

    "str" is incompatible with "list[str]"""".

Candidates: line 1277

filter_engine.py

- 3 errors with message """Cannot access member "append" for type "dict[str, Any]"

  Member "append" is unknown""".

Candidates: line 284, 341, 422

4d2808d

• The linter complains about re.match(str(cfg_forced_modules), module_name) if module_name is None.

3fee6f7

• The changes here are due to adding type hints to the util function parse_did_filter_from_string_fe in utils.py, which is then called in FilterEngine. I added type hints all around to make it easier to understand which types were being passed in FilterEngine, as well as refactoring some stuff where needed for type hinting purposes.

[bari12]

Please rebase

[rdimaio]

Rebased in latest force-push