Add threshold decryption to Anoma

[batconjurer]

Txs on Anoma are to be encrypted with a key posted on chain that has been created by the DKG protocol. In order to decrypt these transcactions, validators must include decryption shares of the tx in their vote extensions. These can be combined to decrypt the transaction.

• Add decryption shares to vote extensions
• Store decryption shares locally until decryption is performed (then they can be removed)
• Empty decryption shares at the beginning of ProcessProposal
• Add verification of decryption shares to VerifyVoteExtension
• Add necessary protocol tx types
• Modify PrepareProposal (maybe?) to apply decryption shares to decrypt txs to be included in next proposal

┆Issue is synchronized with this Asana task by Unito