[WIP] Implement a DCAT-AP converter using RDF

[jahow]

Description

This PR introduces a new metadata converter for the DCAT-AP profile. The Metadata Converter app has been modified to allow handling this new format.

Limitations:

• writing to DCAT-AP is very limited for now
• reading is limited to certain fields of the model (i.e. no extents, no licence/constraints, etc.)

Note that the converter can handle either RDF/XML, JSON-LD or Turtle (both for input and output).

Architectural changes

Added DcatApConverter class in libs/api/metadata-converter

Screenshots

Quality Assurance Checklist

• [x] Commit history is devoid of any merge commits and readable to facilitate reviews
• [x] If new logic ⚙️ is introduced: unit tests were added
• [ ] If new user stories 🤏 are introduced: E2E tests were added
• [ ] If new UI components 🕹️ are introduced: corresponding stories in Storybook were created
• [ ] If breaking changes 🪚 are introduced: add the breaking change label
• [ ] If bugs 🐞 are fixed: add the backport <release branch> label
• [ ] The documentation website 📚 has received the love it deserves