Add support for custom bindgen binaries.

In some cases customized logic is required to generate the expected
bindgen bindings. This adds support for rust_bindgen modules to define a
HostTool module to use instead of bindgen.

Bug: 161816141
Test: New Soong tests pass.
Test: Local test case shows custom_binary module being used for bindgen
      generation.

Change-Id: Id52aec4f25c38206d7e585d8e662be7836aa1d4b