Add prohibited outer forks flag for SAML provider

What does this MR do?

Adds database field to store Group SAML configuration option to prohibit forking outside of GMA group. Frontend and actual behavior will be added in upcoming MRs. Related issue: #34648 (closed)

Migration performance review

saml_providers table contains less than a 1000 records on gitlab.com, so migration will be very fast.

 /chatops run explain select count(*) from saml_providers

Aggregate  (cost=22.66..22.67 rows=1 width=8) (actual time=0.490..0.491 rows=1 loops=1)
  Buffers: shared hit=172 read=1
  I/O Timings: read=0.026
  ->  Index Only Scan using index_saml_providers_on_group_id on saml_providers  (cost=0.27..21.51 rows=458 width=0) (actual time=0.140..0.448 rows=461 loops=1)
        Heap Fetches: 240
        Buffers: shared hit=172 read=1
        I/O Timings: read=0.026
Planning time: 0.676 ms
Execution time: 0.555 ms

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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