Support IAlternateEqualityComparer with FrozenDictionary/Set

[stephentoub]

Closes #27229

Per the discussion in #27229 (comment), for string keys, this only supports an alternate key of ReadOnlySpan<char>... other alternate key types will fail to return a lookup. That's a compromise to not supporting lookups at all.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.