Draft: Refactor YamlProcessor to return a result object

What does this MR do?

Related to #30066 (closed)

Also related to #216444 (closed) and based on top of !34085 (merged).

This refactor simplifies the use of YamlProcessor to better suit the CI Lint use cases and to enable a few new functionalities:

1. make easy to return multiple errors - this is something we have been thinking for a while.
2. return multiple warnings
3. make it easy to use YamlProcessor without raising exceptions

In addition to that this refactoring makes YamlProcessor only responsible for validating the yaml, both statically via Ci::Config class and logically. For the rest it returns a YamlProcessor::Result object which is responsible for presenting the data to the client (errors, warnings and generated configs).

Screenshots

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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