zcash_note_encryption: Add ShieldedOutput::ephemeral_key() -> EphemeralKeyBytes

[str4d]

This replaces the ShieldedOutput::epk() -> &Domain::EphemeralPublicKey which could not be satisfied by output types that did not parse epk.

[nuttycom]

utACK

[codecov]

Codecov Report

Merging #400 (c7c79d2) into master (3915abd) will decrease coverage by 0.06%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master     #400      +/-   ##
==========================================
- Coverage   62.97%   62.91%   -0.07%     
==========================================
  Files          86       86              
  Lines        8417     8407      -10     
==========================================
- Hits         5301     5289      -12     
- Misses       3116     3118       +2     

Impacted Files	Coverage Δ
...h_primitives/src/transaction/components/sapling.rs	76.43% <50.00%> (ø)
zcash_primitives/src/sapling/note_encryption.rs	75.16% <66.66%> (-0.17%)	⬇️
components/zcash_note_encryption/src/lib.rs	68.96% <100.00%> (+1.14%)	⬆️
zcash_client_sqlite/src/chain/init.rs	33.33% <0.00%> (-66.67%)	⬇️
...imitives/src/transaction/components/transparent.rs	58.82% <0.00%> (-1.97%)	⬇️
...imitives/src/transaction/components/tze/builder.rs	43.93% <0.00%> (-1.52%)	⬇️
zcash_primitives/src/transaction/builder.rs	51.47% <0.00%> (-0.60%)	⬇️
zcash_client_sqlite/src/chain.rs	97.65% <0.00%> (-0.08%)	⬇️
zcash_client_sqlite/src/wallet/transact.rs	99.16% <0.00%> (-0.02%)	⬇️
... and 6 more

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