datalake_fdw: the agent engine client (meta/engine_agent)

[MisterRaindrop]

Part of #2008. Letters (A, B0–B7, C, D, E) are the PRs listed there; this is B2.

Scope

src/meta/engine_agent/: the first real IcebergMetaEngine, default in place of the stub (the stub stays for tests).

• C dispatch over a C++ gRPC layer behind the DlErrCode ABI: no exception or longjmp crosses it; agent error detail lands in dl_error_set.
• Stubs generated at build time from B0 (protoc plus the gRPC C++ plugin); grpc++ detected under --enable-datalake-fdw.
• A GUC for the agent address: Unix socket by default, TCP with a token file when set.
• Deadline, cancel polling and a wait event while a call is in flight; handshake on first call; RPC count and time in EXPLAIN ANALYZE.
• Conformance suite: the same scenarios against the stub and against B1. Against B1 it proves the plumbing (handshake, cancel, deadline, agent down, error detail); B3 and B4 turn it fully green.
• CI job: build the B1 jar, start it on a socket, run the suite.

Out of scope

Builtin engine (B4), the proxy.

Depends on

B0; tested against B1.

Acceptance

• Agent down: clear error within the deadline, transaction rolls back, nothing hangs.
• Cancel mid-call returns promptly with no leaked channel.
• Version mismatch refused naming both versions.
• nm -D exports only PostgreSQL entry points (no grpc++/protobuf); builds with and without the gate; Arrow 9 and 17.