Add TIDY_EXTERNAL_VENDOR

Allow external/vendor owners and toolchain developers
to run clang-tidy with external/vendor files,
by setting TIDY_EXTERNAL_VENDOR=1 or true
instead of touching .bp or .go files.

Bug: 244631413
Test: TIDY_EXTERNAL_VENDOR=1 make tidy-soong_subset
Change-Id: I0c903f32eb0e5daa0f8dfa2f6dc892b8f8c4ebcc