[Breaking Change]: fix(client): Update signer method to accept a slice of signers

[wiseaidev]

I have refactored the signer method to accept a slice of signers for multiple signers in a single call. This change simplifies the API by reducing redundancy.

Now we can do:

.signers(&[payer, bid, ask])
.send()

Instead of:

.signer(&payer)
.signer(&bid)
.signer(&ask)
.send()

[vercel]

@wiseaidev is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.

[acheroncrypto]

It's usually better to keep the old method and add a new one instead in cases like this. However, we can do an exception here to keep the API similar to the TS SDK (as it only has signers method).