Allow setting a URI for NFTs

[awrichar]

It's common for a non-fungible token to have an associated URI which points to an immutable and unique identifier for that digital asset (IPFS CIDs are an example of a commonly used identifier).

FireFly should support setting the URI for a given non-fungible token. This also implies work in the token connector to define the behavior and API at that level.

For reference, ERC1155 has the following behaviors surrounding URIs:

• A URI event that should be emitted whenever the URI for a token is set/changed
• A uri() getter that can be used to retrieve the URI for a given token (but by default it returns the same URI for all tokens on the contract)
• A private _setUri() setter that sets the URI (again, assuming the same URI for all tokens and leveraging ID substitution)

These behaviors can be changed/extended as needed to support general use.