build(deps): keep in-process mode off the RPC and HTTP stacks

[anoop-narang]

Closes #522.

In-process mode (liquid-cache-datafusion-local) no longer pulls in the client/server network stack. Its normal dependency graph drops from 387 to 343 crates; arrow-flight, tonic, axum, hyper, h2, reqwest and rustls are all gone.

Changes

• liquid-cache-common: new rpc feature, off by default. It gates pub mod rpc and makes arrow-flight and prost optional. liquid-cache-datafusion-client and liquid-cache-datafusion-server enable it.
• ExecutionMetricsResponse moves to liquid_cache_common::metrics. It is a plain serde struct and the only part of rpc the in-process crates used. extract_execution_metrics stays where it is, with the same signature.
• liquid-cache-datafusion no longer enables object_store's http feature. Nothing in that crate uses it, and it was what pulled in reqwest/hyper/h2/rustls, so gating rpc alone would leave those in place. liquid-cache-datafusion-client calls object_store::parse_url_opts and was relying on this transitively, so it now enables http itself. The server, benchmark and examples already did.
• liquid-cache-common gets tokio with macros as a dev-dependency. Its #[tokio::test]s only compiled before because tonic enabled that feature.
• docs.rs builds liquid-cache-common with rpc, and its README notes the feature.

prost remains in the graph through vortex; that is outside this change.

Breaking changes

• The metrics struct has a new path. liquid_cache_common::rpc::ExecutionMetricsResponse is now liquid_cache_common::metrics::ExecutionMetricsResponse. The serialized JSON is unchanged.
• rpc is now opt-in. Code that uses liquid_cache_common::rpc directly, or through the common re-exports of liquid-cache / liquid-cache-datafusion / liquid-cache-datafusion-local, needs liquid-cache-common = { ..., features = ["rpc"] }.
• No more transitive http. Users of liquid-cache-datafusion or -local who registered http(s):// object stores and got object_store's http feature through this crate need to enable it themselves.

Testing

• Workspace checks. cargo test --workspace --exclude dev-tools gives 236 passed, 0 failed. The test list is identical to main (239 entries). cargo clippy --workspace --all-targets --all-features -D warnings, cargo fmt --check, cargo doc and cargo shear are clean.
• Each crate built on its own. cargo check -p <crate> --all-targets passes for every crate. Workspace feature unification would otherwise hide a missing feature declaration.
• Standalone in-process crate. A crate outside the workspace depending only on liquid-cache-datafusion-local:
  • It builds and runs the examples/datafusion-local.rs flow on nano_hits.parquet; the second run is served from cache and extract_execution_metrics reports correctly.
  • cargo tree shows none of the crates listed above.
• Standalone client crate. A crate depending only on liquid-cache-datafusion-client registers http:// and https:// object stores.
  • With the client's new http line removed, the same build fails at runtime with feature for Http not enabled, so that line is required.
• End-to-end client/server.
  • bench_server with clickbench_client returns correct query results.
  • The example client/server reads an https:// parquet file.
  • The admin /execution_metrics endpoint returns the same JSON keys as before.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #527   +/-   ##
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  Coverage        ?   81.38%           
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  Files           ?       81           
  Lines           ?    13666           
  Branches        ?    13666           
=======================================
  Hits            ?    11122           
  Misses          ?     2216           
  Partials        ?      328           

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