Add flutter cicd template

What does this MR do?

The aim of this merge request is to add a flutter ci template that can be used as a starting point while adding CICD to a flutter project. The goal will be to make it easier for flutter developers to integrate CICD pipelines to automatically test, build and deploy flutter applications.

Screenshots (strongly suggested)

N/A

Sample App template

flutter_ci_cd

Changelog

• Initial release
• Add code quality check
• Add flutter test

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