Add vocabulary for proposal, intent and measure

[scammo]

Summary

I would like to develop federated solutions that allow individuals and organisations to publicly list their offers, which marketplaces can then aggregate. This is inspired by FEP-0837: Federated Marketplace; Status: Draft and the needs of different organisations and individuals. This is not the full implementation of FEP-0837, just the first step.

Problem

Currently, there is no vocabulary included for creating offers in Fedify, and no way to add custom vocabulary.

Proposed Solution

Adding the vocabulary for proposal, intent and measure based on the draft Version of FEP-0837.

Alternatives Considered

It would also suit me if it were possible to add custom vocabulary to Fedify projects like outlined in #207

Scope / Dependencies

Adding the YAML files for proposal, intent and measure, and adding the according vocab-tools snapshots.