[rust-legacy-client] Add confidential mint burn extension

[samkim-crypto]

Problem

There is no support for the confidential mint burn extension in the rust-legacy client.

Summary of Changes

Added support for the confidential mint burn in the rust-legacy client:
329676c: Made a couple of relatively small changes in the account info interface:

• When generating the proof needed to rotate ElGamal public key, the new ElGamal keypair is not actually needed for the proof generation. So I fixed this.
• When generating the proof needed for confidential mint, the proof constructor takes in the current supply amount. I updated this so that it takes in a aes key instead, which can be used to decrypt the current supply. This interface seemed more natural and simpler by the consumers of this function.
• I added the new_decryptable_balance to simplify the rust client a bit.

d5a9575: Added support for the confidential mint and burn in the rust client. I don't think there is anything specifically notable here other than the fact that it is verbose as always to account for all the different variations of how proofs can be provided.

3c9a62b: Added tests rust-client. Nothing notable here either. The logic is in-line with the tests for the confidential transfer and confidential fee extensions.

There is an interface change, so the semver check does seem to complain. Other than that, the rest of the CI checks seem to pass.

[joncinque]

Straightforward and makes sense!