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ARROW-6807: [Java][FlightRPC] Expose gRPC service & client

Creates a utility class that exposes the flight service & client so that multiple services
can be plugged into the same endpoint.
Changes access modifier of FlightClient ctor to package-private
Introduces unit test that binds multiple services to same server and allows multiple
clients to use the services.
Updates gRPC version to 1.24.0

Closes #5597 from rohit7195/Arrow-6807 and squashes the following commits:

882a0220c <Rohit Gupta> ARROW-6807:  Expose gRPC service & client
09cfbafde <Rohit Gupta> ARROW-6807:  Expose gRPC service & client
e62cdfca5 <Rohit Gupta> ARROW-6807:  Expose gRPC service & client
8ec6903eb <Rohit Gupta> ARROW-6807:  Expose gRPC service & client
6016ec471 <Rohit Gupta> ARROW-6807:  Expose gRPC service & client
7743eeab8 <Rohit Gupta> ARROW-6807:  Expose gRPC service & client

Authored-by: Rohit Gupta <rohit@dremio.com>
Signed-off-by: Praveen <praveen@dremio.com>