token-2022: prevent an already configured confidential account to be configured again

[samkim-crypto]

The confidential_transfer::ConfigureAccount, should fail if run on an already configured account as stated in the description. However, this is currently not enforced in the processor logic, allowing users to overwrite the account state.

This PR adds a check in the ConfigureAccount processor logic to prevent accounts from being configured again.

Thank you @joncinque for the catch!

[samkim-crypto]

Added the overwrite flag on initialize_extension. If the flag is set, then initialize_extension will overwrite an existing extension (if it exists) with the default extension state. If the flag is not set, then initialize_extension will return an error if the extension was already initialized.

I ended up just dividing the logic in init_or_get_extension to get_extension and init_extension since the overwrite flag felt unnatural in init_or_get_extension. I hope this change does not break the original readability of the code.

[samkim-crypto]

One alternative to having a boolean flag overwrite is to have two separate functions init_extension_overwrite and init_extension for better readability.

[joncinque]

Looks good to me, but let's see what @CriesofCarrots thinks too, since I think we used to have init and get separated, but there was a good reason to unify them.

Pull request has been modified.

[CriesofCarrots]

Lgtm, I can't remember any specific reason for the old api.

I do find the bool a little unintuitive to read, but I don't want 2 different functions, and can't really think of a reasonable alternative, so let's go with it

[samkim-crypto]

Great! Thanks for the review.