This PR refactors `CredentialHelpers` and removes some unnecessary dependencies as a step of modularization.
The ledger component is almost independent except that it references `MPTokenAuthorize` and `CredentialHelpers.h`, and the latter further references `Transactor.h`. This PR partially clears the path to modularizing the ledger component and decouples `CredentialHelpers` from xrpld.
- Specification: XRPLF/XRPL-Standards#239
- Amendment: `SingleAssetVault`
- Implements a vault feature used to store a fungible asset (XRP, IOU, or MPT, but not NFT) and to receive shares in the vault (an MPT) in exchange.
- A vault can be private or public.
- A private vault can use permissioned domains, subject to the `PermissionedDomains` amendment.
- Shares can be exchanged back into asset with `VaultWithdraw`.
- Permissions on the asset in the vault are transitively applied on shares in the vault.
- Issuer of the asset in the vault can clawback with `VaultClawback`.
- Extended `MPTokenIssuance` with `DomainID`, used by the permissioned domain on the vault shares.
Co-authored-by: John Freeman <jfreeman08@gmail.com>
