Backend: Expose includes links

This is a backend issue to expose the list of files that are included in the main pipeline only.

Following the conversation in &7064 (comment 723983476) files that are been included in a downstream pipeline (include with include) is out of scope for this issue.

Implementation

The breakdown for this feature is as follows:

Work Type	Description	Issue Link
backend	Expose includes link (initial MVC)	👈 You are here
frontend	Change the pipeline editor fixed layout to full-width	#346331 (closed)
frontend	Introduce the feature flag and the empty drawer.	#350721 (closed)
frontend	Pass down the content to the drawer and format it properly	#350722 (closed)
frontend	Remember the open/closed state in local storage	#350723 (closed)
frontend	Add feature discovery popover	#360111 (closed)
frontend	Roll out feature flag	#357219 (closed)
backend	Backend: Add toggle flag to UserCalloutFeatureNameEnum to allow for popover dismissal	#365347
frontend	Store the pipeline editor popovers state in the database	#360635
backend	Add messaging when files can't be fetched due to invalid CI configuration	#360431

Solution for this issue

Include of the main pipeline - this should be an MVC

## .gitlab-ci.yml

include: layer1.yml

We should expose layer1.yml file

1. Expose all types of includes (i.e. local, remote and templates)
2. Expose include with wildcard

Include of includes:

# layer1.yml

include: layer2.yml

3. Expose layer2.yml? and beyond (e.g. layer3,4...)