feat: new features

[Simun17]

Description

Made the following changes:

• fixed bug where omitting the format field in schema registration didn't result in error
• changed the message when sending GET request to the empty Registry
• added config for default header schema id and header schema version
• added error reason to message's header when using Avro format

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (improving code structure without changing external behavior)
• Other (please explain):

Checklist

• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• My code is well documented
• I have read and understood the contribution guidelines

How Has This Been Tested?

Each feature was tested (more details in the PRs to the develop branch)