feat: add onMissingSigner callback

[normanzb]

Change Summary

Previously in order to determine when the signer is missing but required by the button action, it requires the devs to add callback onMissingIdentity to useFarcasterMultiIdentity to determine such event, which is not very straight forward.
(when signer is missing the hook calls into frameStae.signerState.onSignerlessFramePress, which in turns triggers onMissingIdentity callback on the useFarcasterMultiIdentity)

The PR adds a dedicated onMissingSigner callback for useFrame_unstable. The callback will be called when the button action requires a signer but signer is not found.

@michalkvasnicak by looking at the code, it feels to me that adding onTransactionButtonError/onPostButtonError could be confusing, I am worrying the naming could give developers a false impression that all errors happened within transaction/post button action will be caught by the callbacks. (unless we change all errors within button action to also trigger the callbacks?). hence in this PR I renamed the callback name to be more specific.

Merge Checklist

• PR has a Changeset
• PR includes documentation if necessary
• PR updates the boilerplates if necessary

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[normanzb]

@michalkvasnicak do we want to add it into stable useFrame as well to keep it consistent?