feat: add flag to output signer key signature in JSON

[hstove]

This PR adds a new (optional) flag, --json, to the generate-stacking-signature command in stacks-signer. If specified, the CLI will output all the relevant information in JSON form.

The utility of this JSON is that Stacking apps can add a simple "paste JSON" field, which then auto-fills the relevant fields. Because there is a lot of information that must be shared between the signer and stacker, providing this "standard" can help reduce mistakes.

This "paste JSON" functionality is also already implemented in a beta version of Lockstacks:

[codecov]

Codecov Report

Attention: Patch coverage is 30.00000% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 77.83%. Comparing base (c26e00c) to head (37258ee).

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #4580      +/-   ##
==========================================
- Coverage   83.35%   77.83%   -5.53%     
==========================================
  Files         470      470              
  Lines      332256   332271      +15     
  Branches      317      317              
==========================================
- Hits       276953   258616   -18337     
- Misses      55295    73647   +18352     
  Partials        8        8              

Files	Coverage Δ
stacks-signer/src/cli.rs	63.01% <0.00%> (-0.29%)	⬇️
stacks-signer/src/main.rs	15.15% <31.57%> (-20.54%)	⬇️

... and 192 files with indirect coverage changes

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[hstove]

I've updated the log line to not pretty-print the JSON. @xoloki @jcnelson @jferrant would you mind giving this a re-review? 🙏