C#: metadata.Get and GetAll should accept uppercase keys

[jtattermusch]

Fixes #26551.

Similar to #26552, but the implementation is cleaner and it adds tests.

Note that this is mostly a cosmetic change, since according to the gRPC spec, the metadata keys are supposed to be lowercase. But at this point, creating a new Metadata.Entry("KEY", "value") does lowercasing of the key under the hood,
and a subsequent metadata.Get("KEY") won't find the entry. (it's basically a cornercase since there's no real reason to use uppercase metadata keys with gRPC when the spec only allows lowercase.

[jtattermusch]

@StingyJack feel free to also review.

[jtattermusch]

Python bazel test failure is unrelated: https://source.cloud.google.com/results/invocations/94c63c16-0b1f-4445-901f-288ba2e13ca7

[StingyJack]

Its not only uppercase,

TransactionScopeId
CorrelationId
UserActivitySnowflake
etc...

A separate function to wrap a string.Equals is overkill. There is no difference between these changes and the ones I submitted on the other PR when yours just wraps the same string.Equals in a function that looks odd to invoke and that makes use of this.key and key to compare. You think that this is clearer? Seems like you got some pride in your eyes thats affecting your vision.