Scan execution policy: allow selection of branch type

Why are we doing this work

This issue resolves frontend part for scan execution policy.

We need to provide user ability to select branch_type both for rule mode and yaml mode.

Users should only have either branch_type or branches defined in their policy rule. They shouldn't ever have both in the same rule.

Relevant links

Scan execution policy design proposal

See parent epic for yaml mode example.

Implementation plan

Can be broken down to multiple MR's according to plan below:

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policy_rule_branch_selection.vue is already used both by license_scan_rule_builder.vue and security_scan_rule_builder.vue.

It has two options now for specific branches and all protected branches

• Extend policy_rule_branch_selection.vue to provide all options specified by design (all | protected || default).
• Add ability to change list of options for policy_rule_branch_selection.vue
• Allow policy_rule_branch_selection.vue emit changes of branch_type, so it can change yaml mode
• Make sure that both policy and yaml mode work by following rules:
• selecting branch_type modifies yaml mode
• Add validation in scan_execution_policy/lib/from_yaml.js both for new branch_type key and also for possible values: all | protected || default
• Allow policy_rule_branch_selection.vue to accept saved branch_type so it is selected for existing policies and switching between modes

Verification steps

1. Upload a GitLab Ultimate license
2. Navigate to a group/project => Security & Compliance => Policies => New policy => Scan Execution Policy (e.g. gdk group/gdk project
3. Verify changes
4. Switch to yaml mode try some invalid values, make sure that rule still works