chore: Improve proto generation flow

[flemosr]

Moves proto collection and code generation into dedicated envoy-types-tools commands: proto-collect and proto-codegen.

The new flow refreshes committed proto inputs and generated output from clean directories, adds --check modes for CI, and catches changed, deleted, and untracked files before the platform test matrix runs.

Also updates the repo's submodules and documentation for the new maintainer workflow, and bumps the MSRV to 1.88 to match current dependency requirements.

Post-update:

% git submodule status
 40e3bde7f99a5fbddcd6ab4ca811c8e76f603328 tools/submodules/cel-spec (v0.2.0-254-g40e3bde)
 2d24e7e91dcab27ace797932a2a21081cb959502 tools/submodules/client_model (v0.6.1-92-g2d24e7e)
 84e84367f2560cdb47b9bb78fd3e615feb80c3e4 tools/submodules/data-plane-api (remotes/origin/HEAD)
 1dbb1a14e079f78d9214f8e48bf083f32e3ddb96 tools/submodules/googleapis (common-protos-1_3_1-11112-g1dbb1a14e0)
 1d70aa012dc42a5e74a215ce31c1fd84244ce89e tools/submodules/opentelemetry-proto (v0.11.0-321-g1d70aa0)
 92b9a7df69ca9f71bfc492f7a90adf4d36eab569 tools/submodules/protoc-gen-validate (v1.3.0-java-16-g92b9a7d)
 dba9d589def2cd10099a3a64887d859188c2f57a tools/submodules/xds (remotes/origin/HEAD)