Rust: Vendor 3rdparty dependencies.

We've been observing some performance issues using crate_universe on CI.
Therefore, we're moving to vendor the auto-generated BUILD files
in our repository. This should provide a nice speed boost, while
getting rid of the complexity of the "rust cache" job we've been using
when we had a lot of git dependencies.

This PR includes a vendor script, and I'll put up a CI job internally
that runs that vendor script on Cargo.toml and Cargo.lock changes, to check
that the vendored files are in sync.

.bazelrc.internal

@@ -8,4 +8,3 @@ common --registry=https://bcr.bazel.build
 # its implementation packages without providing any code itself.
 # We either can depend on internal implementation details, or turn of strict deps.
 common --@rules_dotnet//dotnet/settings:strict_deps=false
-common --experimental_isolated_extension_usages