Use RustDecider#choose() in for_identifier*() api

[mrlevitas]

Summary

Utilize rust_decider.RustDecider#choose() in get_variant_for_identifier() & get_variant_for_identifier_without_expose()
(instead of rust_decider.make_ctx()/rust_decider.choose() functions)
to emit prometheus metrics.

Notes

1)
If a ctx contains the field that a Feature config, called through get_variant_for_identifier(), needs for bucketing, it will get utilized, since there's no concept of identifier_type in RustDecider#choose() forcing the bucketing to override (we just set ctx[identifier_type] = identifier).

e.g. if a Feature "exp_1" with bucket_val: device_id gets called via:
get_variant_for_identifier("exp_1", identifier="identifier.com", identifier_type="canonical_url")
BUT has the device_id field set on ctx, it will use device_id for bucketing, not
"identifier.com", identifier_type="canonical_url" params.

The only way to establish similar behavior as before would be to clear the other two identifiers in ctx that aren't of the identifier_type param, e.g. clear device_id/user_id from ctx and set canonical_url: "identifier.com" on it.

This is why I deleted 2 test cases..but maybe it is worth clearing the other bucketing fields in ctx?

2)
Removed the overwrite_identifier: bool param/logic in _send_expose()/_send_expose_if_holdout, since we should always use/set the bucket_val: bucketing_value in the v2 exposure event based on the decider event string, not just in some cases, when we had a for_identifier*() "override" .

[mrlevitas]

after discussion, we decided it's ok for the identifier_type/identifier params to be ignored if identifier_type doesn't correspond to the experiment's 'bucket_val' field, and instead lookup the field corresponding to bucket_val in the context and use that for bucketing.
essentially, identifier_type/identifier params are added to the context, but if they don't correspond to the experiment's bucket_val, they will be ignored, since they shouldn't act like an override.

cc @gshipilov