Allow modifying the optional metadata of Confidential Assets

[dbadol]

From FR in https://forum.beam.mw/t/fr-dynamic-asset-text-archive-data/463

Current situation
This is the metadata used to create a Confidential Asset:
https://github.com/BeamMW/beam/wiki/Asset-Descriptor-v1.0

Currently, none of these metadata fields can EVER be modified once the CA has been created.

However, many of the OPTIONAL ones are dynamic and should not be set in stone, on-chain forever. Fields such as website, link to whitepaper, link to logo, link to favicon, etc. should remain open to modification as they depend on off-chain infrastructure (domain names, web hosting, etc.).

Proposed solution
Allow the creator wallet to register new values for the following optional metadata:

OPT_SITE_URL (website link)
OPT_PDF_URL (PDF whitepaper link)
OPT_FAVICON_URL (favicon PNG link)
OPT_LOGO_URL (logo SVG link)
OPT_COLOR (HEX color)

We note, that the previous values of all these fields would still be registered in older blocks. So the whole history of changes would always remain visible on-chain.

Alternative we've considered
The following two fields are also optional ones, and we could decide to allow modifying them A clear and concise description of any alternative solutions or features you've considered. To be discussed...

OPT_SHORT_DESC=CA short description
OPT_LONG_DESC=CA long description

Remarks

• An fee amount could be required for such changes. To be discussed...
• An update to the Beam Asset Minter DApp front end (and also smart contract?) would also be needed.
• Would this change require a hard-fork?...