Implement darkside counterpart for z_getsubtreesbyindex method

[pacu]

What is your feature request?
Implement darkside z_getsubtreesbyindex method

Similarly as it is done with TreeStates we could expose some darkside RPCs that allow tests to add SubTreeRoots to a cache

How would this feature help you?
Currently, ECC SDK can't run the darksidewalletd tests because this method is missing.

-[DarksideTests.AdvancedReOrgTests testReOrgChangesInboundTxMinedHeight] : failed - Failed with error: serviceSubtreeRootsStreamFailed(ZcashLightClientKit.LightWalletServiceError.generalError(message: "there was an attempt to call an unsupported RPC: z_getsubtreesbyindex"))

[LarryRuane]

Which of the following two options is easier, better for you for the darkside gRPC's interface?

• add a single subtree entry (arguments would be index, merkle root hash, end height)
• add an entire list (array) of subtree entries, which would replace the existing array?

Or something else?

[pacu]

Which of the following two options is easier, better for you for the darkside gRPC's interface?

• add a single subtree entry (arguments would be index, merkle root hash, end height)
• add an entire list (array) of subtree entries, which would replace the existing array?

Or something else?

I think the first case will make the interface equal or similar to the TreeState one won't it? I would go for that for the sake of an uniform api