[#13] Fix `TokenMetadataParam`

Description

Problem: The current definition of TokenMetadataParam is not FA2-compliant.

https://gitlab.com/tzip/tzip/-/blob/ed7fbfe859a7c245d067d732c8cb2ce158d63a4f/proposals/tzip-12/tzip-12.md#token_metadata-entrypoint

Solution: modify the definition of TokenMetadataParam to match the layout/annotations defined by the FA2 spec.

Related issue(s)

Resolves #13 (closed)

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