client/wallet: fix decrypt bls key

What

Decrypt tz4 secret key as Signature.Secret_key.t and not Aggregate_signature.Secret_key.t.

Why

https://tezos.stackexchange.com/questions/6405/bls-tz4-signature-with-tezos-ecosystem

Encrypted tz4 (BLS) keys when encrypted were decrypted as aggregate_key type wich is not correct.

This fixes that.

Manually testing the MR

$ octez-client --mode mockup gen keys bls1 -s bls --encrypted
$ octez-client --mode mockup sign bytes 0x03 for bls1

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR