[LSP2] add enum as valueContent (or valueType?)

[CJ42]

When looking at the following JSON Schema for the metadata key LSP8TokenIdType

{
    "name": "LSP8TokenIdType",
    "key": "0x715f248956de7ce65e94d9d836bfead479f7e70d69b718d47bfe7b00e05b4fe4",
    "keyType": "Singleton",
    "valueType": "uint256",
    "valueContent": "Number"
}

It is not clear by looking directly at the schema that there is only 3 possible values that can be stored under this data key.

It would be useful to identify when there are only 3 possible enumerable values.

It should also be flexible enough to allow possible enum values like:

• numbers (e.g: Player level = 1, 2 or 3)
• literal string (e.g: Proposal = Pending, Queued, Active, Expired)
• literal bytesN

Related to:

• Update LSP8 to include value 4 on enum LSP8TokenIdType with for hex encoded ASCII string #143
• [LSP8] Add explanation of hashed tokenId #136
• improve explanation for section LSP8TokenIdType + add tokenId type 4 for string #148