Cleanups, refactoring, and placeholder for TEX addresses

[daira]

This PR includes several other cleanups. Changes related to change strategy will be split out to another PR. fixes #1424

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 58.72093% with 71 lines in your changes missing coverage. Please review.

Project coverage is 63.07%. Comparing base (dd3a557) to head (0b7f60d).

Files	Patch %	Lines
zcash_client_backend/src/data_api/wallet.rs	26.66%	11 Missing :warning:
...ent_backend/src/data_api/wallet/input_selection.rs	28.57%	10 Missing :warning:
zcash_client_backend/src/fees/orchard.rs	0.00%	6 Missing :warning:
zcash_keys/src/address.rs	0.00%	6 Missing :warning:
zcash_primitives/src/transaction/builder.rs	50.00%	5 Missing :warning:
zcash_client_backend/src/fees/common.rs	81.81%	4 Missing :warning:
zcash_client_backend/src/proposal.rs	69.23%	4 Missing :warning:
...mponents/zcash_address/src/kind/unified/address.rs	0.00%	3 Missing :warning:
zcash_client_sqlite/src/lib.rs	25.00%	3 Missing :warning:
..._sqlite/src/wallet/init/migrations/ufvk_support.rs	25.00%	3 Missing :warning:
... and 8 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1423 +/- ## ========================================== + Coverage 63.03% 63.07% +0.04% ========================================== Files 127 127 Lines 14784 14821 +37 ========================================== + Hits 9319 9349 +30 - Misses 5465 5472 +7 ```

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