Provide a way for the user to provide additional bazel configuration.

This is useful to provide additional options aiding in
local development (e.g. warning options), configuration
for local development (e.g. coverage), or providing additional
options needed, such as linkopts.

Described in the best-practice bazel documentation:
https://bazel.build/configure/best-practices#bazelrc-file

Update .gitignore to mention that file, and update it to also
contain other ephemeral artifacts, like the compilation db,
tidy-run output and backup-files.

PiperOrigin-RevId: 616969764

.bazelrc

@@ -42,3 +42,6 @@ build:asan --linkopt -fsanitize=address
 #  bazel run @com_google_fuzztest//bazel:setup_configs > fuzztest.bazelrc
 #
 try-import %workspace%/fuzztest.bazelrc
+
+# https://bazel.build/configure/best-practices#bazelrc-file
+try-import %workspace%/user.bazelrc

.gitignore

@@ -1,3 +1,7 @@
 bazel-*
 .*.sw[a-z]
 __pycache__
+*~
+compile_commands.json
+xls_clang-tidy.out
+user.bazelrc