Improve way to manage default values in case of a Map

[essobedo]

Follow up of #579

Motivation

The current documentation about the support of Map doesn't describe how to define default values, it needs to be added. Moreover #579 adds a sonar violation , it needs to be fixed. Finally tests on failures are missing.

Modifications:

• Rewrites the converter to avoid relying on a regular expression with a negative lookbehind to fix the Sonar violation
• Adds a new config message for the MapConverter in case of invalid format
• Adds unit tests to check that errors are properly thrown in case sub properties are missing and default value is not in the expected format.
• Modifies the example for @ConfigProperty to set defaultValue and describe how to use it properly

[essobedo]

@radcortez Hi, here is the missing part of the previous task

[radcortez]

Thanks!