`rust-analyzer` discoverConfig integration (#3073)

Adds a target that can be used for project auto-discovery by using the
`discoverConfig` settings as described in the `rust-analyzer` user
manual.

Unlike the `gen_rust_project` target, this can be used for dynamic
project discovery, and passing `{arg}` to `discoverConfig.command` can
split big repositories into multiple, smaller workspaces that
`rust-analyzer` switches between as needed. Large repositories can make
it OOM.

At amo, we've used a similar implementation for a while with great
success, which is why we figured we might upstream it. The changes also
include two additional output groups to ensure that proc-macros and
build script targets are built, as `rust-analyzer` depends on these to
provide complete IDE support.

Additionally, the PR makes use of the `output_base` value in `bazel`
invocations. We found it helpful to have tools such as `rust-analyzer`
and `clippy` run on a separate bazel server than the one used for
building. And a `config_group` argument was added to provide the ability
to provide a config group to `bazel` invocations.

An attempt to get codelens actions to work was done as well,
particularly around tests and binaries. They seem to work, but I'm not
100% sure whether the approach taken is the right one.

Closes #2755 .

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Co-authored-by: Krasimir Georgiev <krasimir@google.com>