Update push check and scanning gem to handle exclusions

What does this MR do and why?

This updates the secrets push check and the corresponding scanning engine/gem to handle exclusions of the following types:

• Secrets found in a path that is either specific (e.g. spec/app/project_spec.rb) or a simple glob (e.g. spec/**/*.rb).
• Secrets matching a specific raw value, e.g. dummyfaketoken-1234567890.
• Secrets matching a rule from the default ruleset, e.g. gitlab_pipeline_trigger_token.

Resolves #479308 (closed) and #479309 (closed) (partially).

MR acceptance checklist

I have evaluated this MR against the MR acceptance checklist.

Screenshots or screen recordings

NOTE: All tokens used in the screen recording demo below are made up dummy tokens that was only used for illustrative purposes.

secret-detection-exclusions-merge-request-demo

How to set up and validate locally

• To test the exclusions work as expected, select a project that exist already in your GDK or create a new one.
• Find the project id for the project you have chosen or just created.
• Make sure secret push protection is enabled for the project.
  • Enable pre_receive_secret_detection_push_check FF for the project from Rails console (if not enabled already).
• Ensure to enable the secret_detection_project_level_exclusions FF for the project from Rails console.

> project = Project.find(PROJECT_ID)

> Feature.enable(:secret_detection_project_level_exclusions, project)

• Add a number of new files in the projects, make sure to use a different token for each, and commit them:
  • nested/path/nested-path-exclusion.txt – dummy token you may use: glpat-1234567890123456ABCD
  • path-exclusion.txt – dummy token you may: glpat-12345678900123456789
  • raw-value-exclusion.txt – dummy token you may use: glpat-ABCD1234567890123456
  • rule-exclusion.txt – dummy token you may use: glrt-12345678900123456789
• After commiting the files, try to push to the repository, validate that the push fails with error message highlighting found secrets.
• Navigate to secret detection configuration page: PROJECT_PATH/-/security/configuration/secret_detection.
• Add a new exclusion to match each one of the files based on their types:
  • nested/path/nested-path-exclusion.txt – use path type with value: nested/**/*.txt.
  • path-exclusion.txt – use path type with value: path-exclusion.txt.
  • raw-value-exclusion.txt – use raw value type with value defined as the same dummy token you've used.
  • rule-exclusion.txt– useruletype withvalue` defined as the same rule you have chosen to exclude.
• Try to push again. Validate the push is successful and no error message is displayed.