This CL adds `explicit_build_config_keys` to the metadata for the server
to distinguish between explicit and default configs.
This CL excludes the configs set in `//.gn` from the list, so that
the configs specified by users will be considered as explicit.
Bug: 364971744
Change-Id: I6a3d289d8ca6e054dac691051b53c92ad1ec23c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5853009
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>