Add support for branch exceptions in default rule

What does this MR do and why?

This MR is a follow-up for this comment

This MR request adds possibility for a user to add branch_exceptions for scan result policy for default_rule

It is now only available on project level

Screenshots or screen recordings

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Description	Recording
Project level - no exception option default rule	default rule exceptions.mov

How to set up and validate locally

Enable feature flag

Feature.enable(:security_policies_branch_exceptions)

1. Go to a project
2. Open Secure -> Policies
3. Create new policy or edit existing one
4. Select exceptions in initial state
5. Select scan option in rules section, make sure selected branch type and exceptions are selected
6. Go to a group
   1. Open Secure -> Policies
   2. Create new policy or edit existing one
   3. Make sure there is no exceptions option in UI

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #418743 (closed)