Feat/add append instruct to signatures (#9923) * add append instructions function * add unit tests * tweak comments * Update dspy/signatures/signature.py Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com> Signed-off-by: Karthik Mathur <22126205+mathurk1@users.noreply.github.com> --------- Signed-off-by: Karthik Mathur <22126205+mathurk1@users.noreply.github.com> Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>