Implement `include:component` syntax

With the new decisions added to the blueprint about the directory structure we have all the pieces to start working on an MVC for components.

Problem to solve

As a user I want to create components in a project following the directory structure and guidelines in the blueprint. Then I want to consume these components in my .gitlab-ci.yml.

Details

In this phase it's not required for components to be published to a Catalog. It's sufficient for the user to know the full path of the component to use.

Proposal

Introduce include:component syntax that behaves similarly like other include types. It will automatically support with: inputs once they are available.

include:
  - component: org/my-project@main
  - component: org/my-project/my-component@main

• For org/my-project@main we fetch the file template.yml in the root directory for the HEAD SHA of main branch.
• For org/my-project/my-component@main we fetch the file template.yml in my-component directory for the HEAD SHA of main branch.

Version qualifiers to support are:

• @<sha>
• @<tag_name>
• @<branch_name>

These qualifiers are ordered by precedence based on the decision in the blueprint.

Before merging the resulting YAML from a component we should check that the YAML, which is going to be merged with the main .gitlab-ci.yml, does not contain any of the following top-level keywords: default, variables, stages. These keywords are anti-patterns in the design of components because they would leak settings to other jobs in the pipeline.

Change should be made behind a feature flag.