[Dotnet Analyzer] Add a new rule to validate the async methods always ends with `Async` in their names

Currently we do not have a rule for this purpose.
We should have a rule the enforce the async methods to ends with `Async` in their names in case sometimes we misspell.
The definition of "async method" should be:
1. the methods that has `async` keyword (the most common case)
2. the methods return Task or Task (this could happen if in the implementation there is no `await` keyword used)
3. the methods return AsyncPageable because we do not need `await` keyword when we implement the pageable methods.

No file or test is named. Start by locating the existing .NET analyzer rules and their tests, then verify how async methods are identified for the three cases in the issue. Done means methods matching those definitions are diagnosed unless their names end with `Async`, with tests covering each case.

Written by the indexing model from the issue text.