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 :warning:
signer/src/service/mod.rs	0.00%	30 Missing :warning:
signer/src/service/api/request.rs	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ 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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