Multi transaction and expected public key

[g3co]

Checklist

• App update process has been followed
• Target branch is develop
• Application version has been bumped

This PR includes two new features:

Multi-transaction - the ability to include more than 1 transaction (in TON's terms, more than 1 message) in a single signature to be executed together
Expected public key - clients may send an "expected public key" to Ledger together with transaction data, and Ledger will refuse to sign if the key does not match

⚠️ Note: This PR is a fork of #41, as the ownership of the task has changed. The work has been continued here to move the implementation forward.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.05%. Comparing base (e5c651f) to head (2c55544).
⚠️ Report is 11 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/common/format_bigint.c	66.66%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop      #44       +/-   ##
============================================
+ Coverage    68.42%   91.05%   +22.63%     
============================================
  Files            8       20       +12     
  Lines          247      995      +748     
  Branches         0      116      +116     
============================================
+ Hits           169      906      +737     
- Misses          78       85        +7     
- Partials         0        4        +4     

Flag	Coverage Δ
unittests	91.05% <71.42%> (+22.63%)	⬆️

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