Make communications pluggable in C++ SDK

[charlesbvll]

Issue

Description

The C++ SDK can currently only be used with a gRPC communication channel, this can be limiting for custom communication stacks.

Related issues/PRs

N/A

Proposal

Explanation

In this PR, we make the communication related code separate from the main client logic by creating a new abstract class Communicator and a gRPC implementation of it.

Checklist

• [x] Implement proposed change
• [x] Update the changelog entry below
• [x] Make CI checks pass
• [x] Ping maintainers on Slack (channel #contributions)

Changelog entry

In the C++ SDK, the communication related code is now separate from the main client logic. A new abstract class Communicator has been introduced along side a gRPC implementation of it.

Any other comments?

N/A