Missing GattClient write operation

Some write operations are missing from the current implementation:
- signed write command
- write blob operations:
- prepare write request
- execute (commit) write request

Locate the current GattClient implementation and its existing write operations, then read how requests are encoded and submitted. Use the Bluetooth GATT requirements for signed write commands, prepare write requests, and execute write requests as the behavioral reference; done means all three operations are supported consistently with the existing API.

Written by the indexing model from the issue text.