Helper functions for subscribing to notifications / indications

Currently, an application subscribes to notifications or indications by performing a GATT write procedure on the appropriate CCCD. Knowing which two-byte pattern to write, and remembering to use little-endian, feels a bit too low-level to impose on the application developer.

I suggest we add two helper functions:
```
int
ble_gattc_subscribe_notifications(uint16_t conn_handle, uint16_t cccd_handle,
ble_gatt_attr_fn *cb, void *cb_arg);
int
ble_gattc_subscribe_indications(uint16_t conn_handle, uint16_t cccd_handle,
ble_gatt_attr_fn *cb, void *cb_arg);
```

No files, tests, or entry points are named. Start by locating the existing GATT write procedure used for CCCD subscription and trace how notification and indication values are encoded; done means the proposed helpers provide the requested subscription behavior while accepting the callback and argument parameters.

Written by the indexing model from the issue text.