Features

Add overloads to agrpc::request and agrpc::repeatedly_request for generic RPCs.
Add overload to agrpc::request for starting a client-side unary request. The last argument to this function is a agrpc::GrpcContext instead of a CompletionToken because the function completes immediately.
Add overloads to RPC functions for -Interface versions of stub/service/reader/writer/reader_writer/response_writer.
Add agrpc::GrpcContext.run_completion_queue()/poll_completion_queue() which only process events from the grpc::CompletionQueue. They can be used as a better performing alternative to run()/poll() where the GrpcContext is not used as an Asio execution context.
RPC functions now automatically unwrap std::unique_ptr. (Except stubs which will be addressed in the next release)
Stabilize API of agrpc::CancelSafe. Compared to the previous release it now takes a completion signature as template argument, example: Change agrpc::CancelSafe<bool> to agrpc::CancelSafe<void(bool)>
(experimental) Add agrpc::GrpcStream, an IoObject-like class for handling RPCs in a cancellation safe manner. See the new handle_topic_subscription example on how it can be used to write Golang/Rust select-style code.
(experimental) Replace agrpc::PollContext with a more optimized free function agrpc::run that can be used to run an asio::io_context and agrpc::GrpcContext in the same thread.
(experimental) Add utility function to manually process gRPC tags, useful for writing mocked stubs.

Style

Add pipeline for gRPC 1.16.1.
Use CMake version 3.16.3 for the CMake install test.
Add CMake option to fallback to pkg-config when a dependency cannot be found. This option is for maintainers.
Add GTest to vcpkg dependencies.