Fintekkers-ledger-models

- Support for price requests
- ISSUE_DATE added as a Field

- Some general cleanup (E.g. removing duplicative MeasureMapFieldEntry)
- Initial support for wrappers in Python to simplify the developer experience

The initial release of ledger models. This will be deployment to github's maven repository, as well as crates.io.

The python / javascript packages will be deployment automatically later on.