add bip39_entropy as alternative to mnemonic for signer-service

[holtzman]

Motivation

Offline signers have expressed a desire to store and manage the secret entropy of their account as a 32 byte value rather than as a 24 word mnemonic phrase for consistency with other blockchain wallets that their software manages this way.

Herein we introduce bip39_entropy, a 32 byte, hex-encoded string (64 characters) to the signer-service API to satisfy this need. When outputting account keys, signing txos, or syncing txos for a view-only wallet, users of the signer-service can now chose between providing a 24 word phrase, mnemonic, or a 64 character hex-encoded 32 byte secret, using bip39_entropy.

In this PR

• json rpc request data structures updated to make mnemonic optional and to optionally allow bip39_entropy.
• service calls updated to require exactly one of mnemonic or bip39_entropy and to use whichever is provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 85 lines in your changes are missing coverage. Please review.

Project coverage is 55.38%. Comparing base (ab2af32) to head (e7594fe).
Report is 196 commits behind head on main.

Files	Patch %	Lines
signer/src/service/api/mod.rs	0.00%	49 Missing ⚠️
signer/src/service/mod.rs	0.00%	30 Missing ⚠️
signer/src/service/api/request.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #993      +/-   ##
==========================================
- Coverage   60.12%   55.38%   -4.75%     
==========================================
  Files          88      124      +36     
  Lines       12356    16354    +3998     
  Branches     2010     2815     +805     
==========================================
+ Hits         7429     9057    +1628     
- Misses       3238     5211    +1973     
- Partials     1689     2086     +397     

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[nick-mobilecoin]

main suggestion is to consider matching on a tuple.
I can understand if it feels too dense, but I would still encourage one to move that direction as I think it's more readable than the nested match statements

[sugargoat]

LGTM & thank you @nick-mobilecoin for a great review too!