zcash_client_backend: Extend "send-max" functionality with size limits

[dagny-tracker]

Relevant limits:

• Number of transparent and/or shielded inputs (to enable z_mergetoaddress functionality.
• Size of a transaction (also for z_mergetoaddress but also if we actually want to send the maximum and doing so wouldn't fit in a single transaction, we should have a way to send the maximum that would fit in one tx.

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Created by str4d via Dagny

[linear]

COR-1114