[clang-format] Special handling of spaces for C# code

Summary:
Ensure that there are spaces around braces '{', '}'.

Ensure that there is a space before and after '=>'.

Ensure that 'async' and 'when' are considered as keywords when inserting spaces.

Reviewers: krasimir, MyDeveloperDay

Reviewed By: krasimir

Tags: #clang-format

Differential Revision: https://reviews.llvm.org/D75129