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GH-39335: [C#] Support creating FlightClient with Grpc.Core.Channel (#39348)

as well as Grpc.Net.Client.GrpcChannel by changing our constructor arg to Grpc.Core.ChannelBase which both classes inherit from.

### Rationale for this change

### What changes are included in this PR?

Changing the constructor of C#'s Flight Client to take in a ChannelBase which allows for multiple implementations of gRPC channels to be passed in.

### Are these changes tested?

Existing tests already cover the use but have also manually tested in a separate app (

### Are there any user-facing changes?

No as we're just changing the constructor to take in a parent/base class instead.
* Closes: #39335

Authored-by: John Garland <johnnybg@gmail.com>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>