Define includeSelf template rendering function + consider missing default templates empty

[uvegla]

Towards: giantswarm/roadmap#2456

With config split we move the original include folder of the giantswarm/config repository to shared-configs and customer specific config repositories.

In order to support customer specific include files we add support for the include-self directory. Includes from this folder can be imported by the new includeSelf function.

Keeping include and adding includeSelf is better for error reporting and making it clear where the include is coming from instead of supporting fallbacks to find the file and masking error messages on problems.

Example folder structure:

.
├── default
│   └── <ASSEMBLED FROM SAHRED-CONFIGS>
├── include
│   └── <ASSEMBLED FROM SAHRED-CONFIGS>
│   └── example-1.yaml.template
├── include-self
│   └── example-2.yaml.template
└── installations
    └── <MC_NAME>

And usage in config templates:

# Renders: ./include/example-1.yaml.template
{{- include "example-1" }}

# Renders: ./include-self/example-2.yaml.template
{{- includeSelf "example-2" }}

ℹ️ Additionally, missing default app templates are now considered empty instead of raising and error.

Checklist

• Update changelog in CHANGELOG.md.