RgbResolver: add Send trait bound

[zoedberg]

This PR adds the Send trait bound to the inner field of the AnyResolver struct.

Without this change when trying to create an rgb-lib wallet (which contains an instance of AnyResolver) in tokio::task::spawn_blocking we receive an error, saying that

`(dyn rgb::indexers::any::RgbResolver + 'static)` cannot be sent between threads safely

This was not happening with beta 5, where AnyResolver was an enum

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 3.1%. Comparing base (ab825ab) to head (af4c130).
Report is 2 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff          @@
##           master   #243   +/-   ##
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  Coverage     3.1%   3.1%           
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  Files          14     14           
  Lines        1729   1729           
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  Hits           53     53           
  Misses       1676   1676           

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[dr-orlovsky]

I think Send has to be added as a generic bound to the RgbResolver trait itself.

[zoedberg]

I think Send has to be added as a generic bound to the RgbResolver trait itself.

It makes sense. Done