interface: Demote all signers during onchain invoke

[joncinque]

Problem

There's just too many ways for signers to be potentially abused during transfer hooks.

Summary of changes

Demote all accounts to non-signer before invoking the transfer hook program. Let me know if you think the test is useless, I just wanted to be sure that the code was correct.

At the same time, remove cdylib target for the interface crate, and remove the lib target for the program crate. NOTE: can't do the second one

cc @tiago18c

[buffalojoec]

The change itself is fine, but we have a bit of a confusing dependency tree going for transfer-hook, and the leaf (tlv-account-resolution) actually has a method that's supposed to do some of this work under the hood.

https://github.com/solana-program/libraries/blob/788a65685ccfdfd4d33af752b9aadf739626d113/tlv-account-resolution/src/state.rs#L35

Maybe it's best to include this change there instead?

[joncinque]

My thinking was that TLV account resolution could be used for other use-cases, outside of transfer hook programs, so that it would be better to fix it only at the level that we were worried about, which was transfer hooks.

After writing it out though, I realize that's kind of silly -- we can always add new functions in TLV account resolution later if needed.

[buffalojoec]

After writing it out though, I realize that's kind of silly -- we can always add new functions in TLV account resolution later if needed.

Agreed, we can just hoist deescalate_account_metas out of that library if we ever need to. For now, might as well pile on, since the name is kinda implied already.