Add `UnifiedIncomingViewingKey` struct #5024

Job	Run time
Build target wasm32-wasi	1m 18s
Clippy (MSRV)	2m 1s
Check ZFuture build on Linux	24s
Latest build on ubuntu-latest	1m 17s
Check ZFuture build on macOS	1m 0s
Bitrot check	1m 35s
Clippy (beta)	12s
Latest build on windows-latest	4m 21s
Check ZFuture build on Windows	1m 45s
Latest build on macOS-latest	3m 7s
Code coverage	17m 56s
Rustfmt	15s
Intra-doc links	2m 24s
Test on Linux	9m 23s
UUID validity	3s
protobuf consistency	1m 42s
Test Orchard on Linux	6m 51s
Test NU6 on Linux	9m 17s
Test on macOS	21m 0s
Test Orchard on macOS	12m 38s
Test on Windows	13m 1s
Test Orchard on Windows	10m 2s
Test NU6 on Windows	13m 7s
	2h 14m 39s

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