fix(csharp/FieldMask): Support map fields in c# FieldMask

[ahmednfwela]

fixes #11989

This PR adds support to referencing keys in a map field via field masks and introduces tests that used to fail before.

related issues:

• FieldMask JSON serialization improvement proposal #12524

AIP: https://google.aip.dev/161#map-fields

[ahmednfwela]

Note for reviewers:

This PR fixes two related issues

1. Merging FieldMasks that target a map field (referenced in Field Mask merge with a map fails #11989)
2. Referencing a specific key for a map in a FieldMask (referenced in AIP 161 and FieldMask JSON serialization improvement proposal #12524)

The tests might seem repetitive, but they cover all possible branches of the introduced code, that's why I split them into a separate test class.

Future work:

1. Support parsing back ticks: currently we do a simple split by . to determine paths, but in the future we can add support for backticks for invalid paths, e.g.:

// Valid field masks: reviews, reviews.smith, reviews.`John Smith`, reviews.`Dr. John Smith`
map<string, string> reviews = 2;

Also cc @jskeet since he worked on the original implementation

[jskeet]

I'm not going to have time to review this PR this year, I'm afraid.

[ahmednfwela]

I'm not going to have time to review this PR this year, I'm afraid.

That's ok, but would it be fine if you approved running the tests?

[jskeet]

I'm not going to have time to review this PR this year, I'm afraid.

That's ok, but would it be fine if you approved running the tests?

Yup, will do that now.