Metadata editor / add a configuration to define mandatory keywords from a thesaurus

[josegar74]

The directive gnKeywordSelector has the required attribute to display the field as mandatory, but it was not used.

This change request adds an attribute in the thesaurus element of the metadata editor configuration to allow to define a thesaurus as mandatory:

<thesaurus 
  key="local.theme.pg-privacy" fieldset="false" maxtags="1"  transformations="to-iso19139-keyword"
  mandatory="true" />

Checklist

• I have read the contribution guidelines
• Pull request provided for main branch, backports managed with label
• Good housekeeping of code, cleaning up comments, tests, and documentation
• Clean commit history broken into understandable chucks, avoiding big commits with hundreds of files, cautious of reformatting and whitespace changes
• Clean commit messages, longer verbose messages are encouraged
• API Changes are identified in commit messages
• Testing provided for features or enhancements using automatic tests
• User documentation provided for new features or enhancements in manual
• Build documentation provided for development instructions in README.md files
• Library management using pom.xml dependency management. Update build documentation with intended library use and library tutorials or documentation

[juanluisrp]

LGTM. I've added some updated documentation too.