Add graphql_schema lefthook check

What does this MR do?

This MR adds a graphql_schema section into lefthook.yml that runs the following when any Ruby file is updated and a git push is attempted:

bundle exec rake gitlab:graphql:check_schema

Examples

$? is the exit code where 0 is good and will pass and 1 is not-good and will fail:

No errors

$ bundle exec rake gitlab:graphql:check_schema ; echo $?
GraphQL schema is up to date
0

Errors

$ bundle exec rake gitlab:graphql:check_schema ; echo $?
##########
#
# GraphQL schema is outdated! Please update it by running `bundle exec rake gitlab:graphql:schema:dump`.
#
##########
1

Links up with gitlab-development-kit!1825 (merged).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team