MESH-1853/ add field for external id to settlements

[JMoore96]

changelog

modified API

• added Instruction with memo
• updated storage and events

[codecov]

Codecov Report

Merging #1280 (d5367f4) into develop (609295d) will decrease coverage by 0.11%.
The diff coverage is 12.14%.

@@             Coverage Diff             @@
##           develop    #1280      +/-   ##
===========================================
- Coverage    44.03%   43.92%   -0.12%     
===========================================
  Files          141      141              
  Lines        29686    29788     +102     
===========================================
+ Hits         13073    13083      +10     
- Misses       16613    16705      +92     

Impacted Files	Coverage Δ
pallets/weights/src/pallet_settlement.rs	5.38% <0.00%> (-0.66%)	⬇️
pallets/settlement/src/lib.rs	46.80% <12.64%> (-2.41%)	⬇️
pallets/sto/src/lib.rs	24.73% <100.00%> (+0.16%)	⬆️
primitives/src/asset_identifier.rs	97.92% <0.00%> (-0.42%)	⬇️

[Neopallium]

Once the memo is changed to a fixed-length array, this PR should be finished.

[adamdossa]

• We should add a with_memo version of add_and_affirm_instruction
• We can mark the old add_instruction and add_and_affirm_instruction as deprecated