floss: Allow sending a GATT server request response with btclient

Allow btclient users to manually send a response to GATT server
attribute read and write requests.

Bug: 328813054
Test: m -j && manually with btclient:
Add and register services. Connect with NRF Connect.
Send read/write requests with NRF Connect and send a response after each
request.
Observe that requests are processed.
Flag: Exempt, Floss-only change

Change-Id: I0840403d91b88dde2a723a02ce12de8a7a2fe5a1