Support larger discovery characterisrics maximum buffer size

[tmpfs]

Hi,

I just hit the panic here:

https://github.com/embassy-rs/nrf-softdevice/blob/master/nrf-softdevice/src/ble/gatt_client.rs#L179

And I can't see any way to increase the maximum number of characteristics when discovering services as it's hard-coded in const DISC_CHARS_MAX.

The problem does not exist when I have an att_mtu of 23 but when i increase the att_mtu to 247 which i need for more throughput reading some of the GATT characteristics then the problem appears.

In my case the packet contained 11 characteristics with an att_mtu of 247 so I have bumped it to 12 behind the discovery-chars-12 feature flag (taking inspiration from the evt-max-size-256 feature flag - which I also need).

I am tempted to do the same for DISC_DESCS_MAX but perhaps that's my OCD with consistency and symmetry so perhaps you can advise if you would prefer this or not?