Add support for payment-v2 memo field

[vihu]

Enables support for specifying a uint64 memo for a payment-v2.

Notes:

• Introduces allow_payment_v2_memos boolean chain variable
• If the var is not set, existing payment-v2s will have memo=0 as default
• If the var is set, as long as the memo lies between 0 - 2^64 it will be considered valid
• Add some test cases to verify said logic
• Update and refactor payment-v2 txn validation

TODO:

• Switch proto to master once proto#72 lands.

[codecov]

Codecov Report

Merging #768 (a3b7118) into master (ea3a112) will decrease coverage by 0.05%.
The diff coverage is 91.30%.

❗ Current head a3b7118 differs from pull request most recent head aa3a0a6. Consider uploading reports for the commit aa3a0a6 to get more accurate results

@@            Coverage Diff             @@
##           master     #768      +/-   ##
==========================================
- Coverage   71.53%   71.48%   -0.06%     
==========================================
  Files          97       97              
  Lines       14009    14042      +33     
==========================================
+ Hits        10022    10038      +16     
- Misses       3987     4004      +17     

Impacted Files	Coverage Δ
src/transactions/v1/blockchain_txn_vars_v1.erl	62.85% <50.00%> (-0.11%)	⬇️
src/transactions/v2/blockchain_payment_v2.erl	91.30% <87.50%> (-2.45%)	⬇️
src/transactions/v2/blockchain_txn_payment_v2.erl	95.77% <96.96%> (+0.77%)	⬆️
src/ledger/v1/blockchain_ledger_snapshot_v1.erl	47.42% <100.00%> (ø)
src/blockchain_gateway_cache.erl	55.81% <0.00%> (-9.31%)	⬇️
src/handlers/blockchain_sync_handler.erl	92.85% <0.00%> (-1.79%)	⬇️
src/ledger/v1/blockchain_ledger_v1.erl	80.21% <0.00%> (-0.10%)	⬇️
src/transactions/blockchain_txn.erl	82.35% <0.00%> (+0.14%)	⬆️

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[Vagabond]

Did we want to land this?

[vihu]

Did we want to land this?

I think this is good to land despite differing slightly from HIP26, but the author seems okay with this implementation as well.