Generalize auth/type.StdFee

[alessio]

A new Fee interface is made available in the top level types package.
auth.StdFee implements such interface. User defined auth module can
now define their own custom fee types.

Work carried out in the context of the following issues:

• Decouple auth module from other SDK top-level core packages #4488

• Remove X package dependency - beside exported package #4487

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added a relevant changelog entry: clog add [section] [stanza] [message]

• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
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• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[rigelrozanski]

I would rather see abstractions of the fee logic to auth and have a generic FeeProcessor interface in types which auth could fulfill. This way the stdFee and all associated logic would live in auth and be readily swap-able with other auth logic sets (edited)
All sort of blockchains are going to want to use radically different fee-mechanisms

[codecov]

Codecov Report

Merging #4509 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4509      +/-   ##
==========================================
+ Coverage   53.23%   53.24%   +0.01%     
==========================================
  Files         259      259              
  Lines       16203    16205       +2     
==========================================
+ Hits         8625     8629       +4     
+ Misses       6932     6930       -2     
  Partials      646      646

[alexanderbez]

Ref: #4512

Second @rigelrozanski here. It seems like we want Authenticator and FeeProcessor interfaces defined in types/ which x/auth/types fulfills.

[alexanderbez]

@alessio bump or close por favor.