Add more proptest strategies for sapling shielded data

[oxarbitrage]

In #2020 we added support for sapling shielded data in V5 transactions and we added a good amount of test coverage for the new feature.

In the file zebra-chain/src/sapling/tests/prop.rs we modify the generated shielded data in several tests instead of having more strategies. There are TODO comments at each spot we are doing this:

// TODO: modify the strategy, rather than the shielded data

The solution is to generate different strategies that have what we need, for example instead of just shielded_v4 and shielded_v5 strategies we can have shielded_v4_no_spends, shielded_v4_no_outputs, shielded_v4_full, shielded_v4_empty, etc.

This is currently low priority.

[oxarbitrage]

Not needed right now and there are TODO comments in the code. We can always reopen or open a new issue for it.