bazel: re-organize bazelrc (#15522)

Replaced ci.bazelrc and v8-ci.bazelrc by custom configs inside the main
.bazelrc file. As a result, github workflows setup is simplified down to
a single '--config=<foo>' flag usage.

Moved the build metadata flags to config=ci.
Added custom tags metadata to help differentiate invocations based on
workflow (bazel vs v8) and os (linux/macos/windows).

Enabled users to override the default values in .bazelrc by using a
user.bazelrc file locally.
Added user.bazelrc to gitignore.