Add Ci::Pipeline source for DAST site validation

What does this MR do?

this merge request adds a new Ci::Pipeline source (ondemand_dast_validation) for dast runner validation. this was necessary because we want to differentiate between the trigger for each situation (scans and validations).

Related Issues

• DAST On-Demand: Move DAST site validation to runner

Context

• Discussion Thread

How to setup and validate locally (strongly suggested)

• enable feature flag Feature.enable(:dast_runner_site_validation)
• create a project
• create a dast site profile
• validate your dast site profile

Screenshots

Feature

Pipeline

Does this MR meet the acceptance criteria?

Conformity

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• 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.)
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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

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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
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