Add security policies edit page

What does this MR do?

For: #335453 (closed)

This MR adds:

• New route (/:group/:project/-/security/policies/:id/edit) to edit scan execution policies
• New #edit view in Projects::Security::PoliciesController to allow for existing policies to be edited
• New service (Security::SecurityOrchestrationPolicies::FetchPolicyService) to retrieve orchestration policies by type + name

To allow users to edit existing policies stored in a policy management project, within the policy editor.

For full background on this feature, see: &5362 (closed)

Screenshots or Screencasts (strongly suggested)

Policy editor does not render, but the policy data does get passed in to the component. I assume that there are additional changes needed on the Vue frontend, and those can be done by a frontend engineer in a follow-up MR since this is behind a feature flag.

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.