add max supply global state for inflation asset

[cymqqqq]

Description:

In rgb20 Inflation asset iface, there are issuedSupply and maxSupply in the global state, here I try to add the maxSupply method for the rgb-interface.

[dr-orlovsky]

Max supply can be absent - depending on specific RGB20 interface used. For instance, it is absent for non-inflatible assets. Thus, this method should default to the issued supply value if the max supply is not found

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Please upload report for BASE (master@baafff6). Learn more about missing BASE report.

Files	Patch %	Lines
src/rgb20/wrapper.rs	0.0%	12 Missing ⚠️
src/rgb20/issuer.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             master      #5   +/-   ##
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  Coverage          ?   39.9%           
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  Files             ?      17           
  Lines             ?    1887           
  Branches          ?       0           
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  Hits              ?     753           
  Misses            ?    1134           
  Partials          ?       0           

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[dr-orlovsky]

one last small thing and I can merge this

[dr-orlovsky]

We need this not to do an unnecessary secondary call to global if we have maxSupply in place

Suggested change

	.unwrap_or(
	self.0
	.unwrap_or_else(|| self.0