docs: add metadata standard for Stablecoins

[jamaljsr]

I've added a new document which defines the asset metadata standard that Terminal will use to extract rich data for assets and display on the website. I would like to gather feedback from the team and wider community before finalizing this.

Summary

This document describes the metadata specification for stablecoins minted on the Taproot Assets Protocol. When minting assets, issuers can include additional metadata about the asset which will be stored and synchronized with universes. Specifying the metadata using this standard allows applications like Terminal to pull in rich data and display them in the UI. The specification outlined below focuses solely on stablecoin assets that will not have a fixed supply. It is expected that there will be multiple issuance and burn events using the same Group Key to ensure fungibility between asset tranches.

[coveralls]

Pull Request Test Coverage Report for Build 15049096866

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 70 unchanged lines in 8 files lost coverage.
• Overall coverage decreased (-0.06%) to 36.785%

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Files with Coverage Reduction	New Missed Lines	%
asset/group_key.go	2	57.89%
tappsbt/create.go	2	26.74%
asset/mock.go	6	64.86%
tapchannel/aux_invoice_manager.go	6	80.51%
tapgarden/caretaker.go	6	68.31%
address/address.go	8	67.47%
address/mock.go	14	89.54%
asset/asset.go	26	46.84%

Totals
Change from base Build 15041651130:	-0.06%
Covered Lines:	26494
Relevant Lines:	72024

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💛 - Coveralls

[ffranr]

LGTM 👍

[guggero]

Nice!