Add a z_shieldtransparent RPC method.

[nuttycom]

This RPC method should enable the shielding of all funds from:

• ANY_TADDR with the semantics that all funds from all legacy taddrs are selected (linking transparent information on-chain) - in this case, the user should also specify a unified account ID as a destination.
• An individual legacy taddr to shield from (to inhibit linking of taddrs if the user chooses)
• A unified account ID and optional diversifier index to shield funds from.

If ANY_TADDR is selected, or no diversifier index is specified, the user should also be required to provide the AllowLinkingAccountAddresses privacy weakener to ensure they understand the repercussions of this action. The default policy here is AllowRevealedSenders.

This could also potentially subsume z_shieldcoinbase.