Refactor YamlProcessor to always 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). In short, this also breaks the many responsibilities that YamlProcessor has today.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content