This abstracts the base Transport handler to have a MessageHandler
component and allows us to generalize both JSON-RPC 2.0 for MCP (or an
LSP) and DAP format.
This should allow us to create clearly defined clients and servers for
protocols, both for testing and for RPC between the lldb instances and
an lldb-mcp multiplexer.
This basic model is inspiried by the clangd/Transport.h file and the
mlir/lsp-server-support/Transport.h that are both used for LSP servers
within the llvm project.
Additionally, this helps with testing by subclassing `Transport` to
allow us to simplify sending/receiving messages without needing to use a
toJSON/fromJSON and a pair of pipes, see `TestTransport` in
DAP/TestBase.h.
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