Rename asset-related entries in `shared/base.yaml`

[Bronek]

With the introduction of MPT to XRP Ledger we have added support for a new kind of tokens. They are similar to IssuedCurrency but more flexible (e.g. enhanced authorization mechanism). I would also like to recognize that, what is currently called IssuedCurrency is not necessarily currency either - it is also a token. The all-encompassing name Currency is not a good fit for both XRP and tokens.

What follows is a proposal of cleanup in terminology used in shared/base.yaml:

• #/components/schemas/Currency rename to Asset . It is a more general term which corresponds to both XRP and tokens
• #/components/schemas/IssuedCurrency rename to IOU. This is a technical term used for old-style issued currencies, or old-style tokens
• #/components/schemas/MultiPurposeToken (added in Add OpenAPI spec for SingleAssetVault #46) rename to MPT, which is just a commonly used abbreviation

The above proposed new naming would align the specification with the language used in both unit tests and comments in rippled , making it easier to navigate both.

Alternative names, also aligned with rippled would be:

• #/components/schemas/Currency rename to Asset
• #/components/schemas/IssuedCurrency rename to Issue
• #/components/schemas/MultiPurposeToken rename to MPTIssue

These names correspond with the types used internally in rippled hence are probably more brittle (renaming types is relatively common).

Either way, I am not proposing renaming #/components/schemas/XRP