Add `UnifiedIncomingViewingKey` struct #5026

Job	Run time
Build target wasm32-wasi	1m 23s
Test on Linux	9m 11s
Test Orchard on Linux	6m 51s
Latest build on ubuntu-latest	1m 27s
Check ZFuture build on Linux	23s
Test NU6 on Linux	9m 15s
Latest build on windows-latest	3m 13s
Test on macOS	23m 6s
Latest build on macOS-latest	3m 0s
Bitrot check	1m 42s
Test Orchard on macOS	15m 0s
Test on Windows	13m 25s
Clippy (MSRV)	1m 56s
Test Orchard on Windows	10m 22s
Test NU6 on Windows	14m 0s
Clippy (beta)	14s
Code coverage	17m 21s
Intra-doc links	2m 29s
Rustfmt	14s
Check ZFuture build on macOS	1m 7s
UUID validity	3s
protobuf consistency	1m 39s
Check ZFuture build on Windows	1m 33s
	2h 18m 54s

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