extensions: `mintable_templates` & `mutable_attributes`

[marc0olo]

based on #148

introduces:

• mintable_templates extension
  • edition limit can be defined on template level and decreased afterwards
• mintable_templates_limit extension (to limit the max amount of templates that can exist)
  • template limit can be decreased afterwards
• mutable_attributes extension to standardize how mutable attributes are handled

[marc0olo]

@mradkov I provided the updates in regards to edition limit and we now allow to change (decrease) afterwards

@jyeshe let's discuss the immutable_metadata_url topic publicly in the forum and involve as many people as possible in this discussion. I don't think we can and should find a consensus here in this proposal. though I think the current approach is the best and right approach :-)

[marc0olo]

had a discussion with @jyeshe about metadata and will propose new changes here. we will allow URL and MAP on template level to be flexible again

note:

• reference implementation provided by me will only propose URL

[marc0olo]

here we go again 🙈 ... I hope all points are covered now 🤞

[marc0olo]

finally =)