Get branches from rules using rule_index for security policy

What does this MR do?

Addresses #337537 (closed)

The schedule rule type in Security Orchestration Policies accepts branches field for which the policy is expected to be applied. But currently, the scheduled scan execution does not consider branches and the policy is applied only to the default branch of the project.

Screenshots or Screencasts (strongly suggested)

Policy with branches

Branches in project

Pipeline

How to setup and validate locally (strongly suggested)

1. Clone and create a project from https://gitlab.com/sashi_kumar/security-orchestration-policies
2. Enable security_orchestration_policies_configuration for the target project for which the scan policies should be enabled by doing:

Feature.enable(:security_orchestration_policies_configuration, Project.find(<target_project.id>))

1. Go to Security & Compliance -> Scan Policies for the target project
2. Select Edit policy project and select the policy project (security-orchestration-policies) that was created in step 1 and click save
3. Make sure the branches mentioned in the policy exist in the project
4. Wait for few minutes (>10mins) to see the scheduled scan triggered for the branches specified.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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