feat(nns): Support disbursing maturity to an AccountIdentifier

[jasonz-dfinity]

Why

Currently, DisburseMaturity supports disbursing into any icrc1 account. However, we still have valid use cases beyond icrc1 account, for example for disbursing into a CEX account, which typically does not use icrc1 account but a 32-byte account identifier.

What

• Add to_account_identifier as a new field in the DisburseMaturity command input
• Change account_to_disbures_to in the storage type into a oneof destination with the previous account_to_disbures_to as one variant.
• Implement Destination::try_new to handle the 3 cases - (a) no account or account identifier (use caller), (b) account, and (c) account identifier
• Implement the conversion from Destination to AccountIdentifier for the finalization (for minting ICPs).
• Implement the conversion from Destination to Option<Account> and Option<AccountIdentifier> for the fields in "full neuron"
• Perform a similar validation for neuron management proposals

Backward Compatibility

The main concern of the backward compatibility is that there can already be disbursements prior to this PR, and they do not use the oneof destination but the previous schema with only account_to_disbures_to. The oneof is designed to be compatible with the previous schema, and the test_deserialize_maturity_disbursement_from_old_format test is added to make sure that the decoding actually works, at the Storable level.

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Done

[oggy-dfin]

TLA instrumentation changes LGTM, thanks!

[andrew-lee-work]

LGTM.