Transfer helper: support signed transfer hooks

[grod220]

Follow up from #233

At the moment the invoke_transfer_checked() function is indiscriminately adding signed accounts to the CPI instruction. Normally this is fine, but when a transfer hook is involved, a bug is apparent. It mistakenly gets added to the list of accounts and then later again as it's handled by add_extra_accounts_for_execute_cpi().

This PR fixes this bug by parsing multisig accounts specifically versus signed accounts generically.

[grod220]

The key variable change. When making an extra transfer hook account signed, it previously caused the tests to fail.

[buffalojoec]

Just to make sure I understand, the problem is that if you mark an extra transfer hook account as a signer, it mistakenly gets added as a multisig signer as well?

So, for example, if we have two multisig signers and a signed extra hook account:

Instruction: "TransferChecked"
    * [w] Source
    * [ ] Mint
    * [w] Destination
    * [ ] Authority (multisig account)
    * [s] Multisig signer #1
    * [s] Multisig signer #2
    * [s] Extra transfer hook meta
    * [s] Extra transfer hook meta
    * [ ] Validation account
    * [ ] Transfer hook program

But where does this throw?

[grod220]

By adding all signed additional accounts to the signer pubkeys vec, you'd get this base instruction:

Instruction: "TransferChecked"
    * [w] Source
    * [ ] Mint
    * [w] Destination
    * [ ] Authority (multisig account)
    * [s] Multisig signer #1
    * [s] Multisig signer #2
    * [s] Extra transfer 1 hook meta
    * [s] Extra transfer 2 hook meta

and then add_extra_accounts_for_execute_cpi() would append the extra accounts:

    * [s] Extra transfer 1 hook meta
    * [s] Extra transfer 2 hook meta
    * [ ] Validation account
    * [ ] Transfer hook program

So the resulting call would look like this:

Instruction: "TransferChecked"
    * [w] Source
    * [ ] Mint
    * [w] Destination
    * [ ] Authority (multisig account)
    * [s] Multisig signer #1
    * [s] Multisig signer #2
    * [s] Extra transfer 1 hook meta
    * [s] Extra transfer 2 hook meta
    * [s] Extra transfer 1 hook meta
    * [s] Extra transfer 2 hook meta
    * [ ] Validation account
    * [ ] Transfer hook program

[buffalojoec]

So the resulting call would look like this:

Wow, I guess we really didn't test this thing with multisigs and transfer hooks. 😂 😂

[joncinque]

Just a few comments, looks good otherwise!

[joncinque]

A couple more things on my side, then this can go in!

[joncinque]

Looks great!

[buffalojoec]

Lgtm, but I still think we could avoid the vector.