Add child identity protocol fee

[adamdossa]

changelog

new features

• Adds a protocol fee for creating new child identities. Initially set to 0

[codecov[bot]]

Codecov Report

Merging #1523 (bd90ad6) into develop (8827b1d) will increase coverage by 0.00%. The diff coverage is 50.00%.

@@           Coverage Diff            @@
##           develop    #1523   +/-   ##
========================================
  Coverage    58.45%   58.46%           
========================================
  Files          142      142           
  Lines        31415    31413    -2     
========================================
+ Hits         18365    18366    +1     
+ Misses       13050    13047    -3     

Files Changed	Coverage Δ
pallets/common/src/protocol_fee.rs	33.33% <ø> (ø)
pallets/identity/src/keys.rs	87.22% <50.00%> (+0.10%)	:arrow_up:

... and 4 files with indirect coverage changes