signed extensions: metadata type generation using reflect

[nicksinch]

Detailed description:

• Aims to remove the necessity of writing manually a Metadata() function for each signed extension
• This is achieved using a dynamic map that stores the ids of metadata types which is initialized prior starting to build the metadata of the extrinsic
• Metadata types are generated using reflect package when iterating recursively the extensions of the signedExtra type

Which issue(s) this PR fixes:
Fixes: part of #191

Checklist

• Documentation added
• Tests updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (71d0279) 78.37% compared to head (587e380) 78.51%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #304      +/-   ##
===========================================
+ Coverage    78.37%   78.51%   +0.14%     
===========================================
  Files          159      159              
  Lines        10432    10500      +68     
===========================================
+ Hits          8176     8244      +68     
  Misses        1895     1895              
  Partials       361      361              

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