add smoke test (uses darkside wallet server)

[LarryRuane]

Add "smoke" test. This is analogous to zcash's qa/zcash/smoke_tests.py (but the details are completely different). Our existing tests don't exercise the gRPC interface; this test addresses that deficiency. It doesn't exercise every gRPC (it could be extended), but is still a significant improvement in test coverage.

I've wanted to write this PR for a long time, but the tipping point was #505 (not yet merged at this time)-- that PR changes the gRPC interface internals (not functional behavior) quite a bit, and I didn't have a way to test those changes except manually. I'd like to get this PR merged first, before merging #505.

This test could be added to CI (it takes about 9 seconds to run), but for now, just run it manually with no arguments. It does not require running a zcashd instance (since it uses the darkside wallet test infrastructure). It does require:

• grpcurl
• jq (but this dependency could be eliminated pretty easily).

[LarryRuane]

Force pushed to add copyright comment to new file smoke-test.bash.

[LarryRuane]

The following files are generated, so don't need review:

• walletrpc/darkside.pb.go
• walletrpc/darkside_grpc.pb.go
• docs/rtd/index.html

[LarryRuane]

Force pushed to test donation address (#506 just merged)

[y4ssi]

utACK, but suggest improving dependency documentation in smoke-test.bash

[y4ssi]

The script relies on external tools (grpcurl, jq, curl, timeout), but it doesn't explicitly document installation or pre-checks.

Suggestions:

1. Add a "Requirements" section at the top listing dependencies and installation commands (e.g., apt-get install grpcurl jq curl).
2. Include a pre-check to verify dependencies before execution:

   command -v grpcurl >/dev/null 2>&1 || { echo "Error: grpcurl not installed."; exit 1; }

3. Improve error handling to provide clearer messages when dependencies are missing.