Characteristic descriptor discovery

[pan-]

This pull request provides:

• API to make characteristic descriptor discovery
• minor improvements for FunctionPointerWithContext

A characteristic descriptor discovery can be made on two levels:

• directly from a characteristic using the discoverDescriptor member function

void whenCharacteristicDescriptorDiscovered(const CharacteristicDescriptorDiscovery::DiscoveryCallbackParams_t* params) {
   // do something with these
   (void) params->characteristic;
   (void) params->descriptor;
}

void whenCharacteristicDescriptorDiscoveryTerminate(const CharacteristicDescriptorDiscovery::TerminationCallbackParams_t* params) {
   // do something with these
   (void) params->characteristic;
   (void) params->status;
}

DiscoveredCharacteristic foo;
foo.discoverDescriptors(
    whenCharacteristicDescriptorDiscovered, 
    whenCharacteristicDescriptorDiscoveryTerminate
);

• using the GattClient low level interface

ble_error_t discoverCharacteristicDescriptors(
        const DiscoveredCharacteristic& characteristic,
        const CharacteristicDescriptorDiscovery::DiscoveryCallback_t& discoveryCallback,
        const CharacteristicDescriptorDiscovery::TerminationCallback_t& terminationCallback);

bool isCharacteristicDescriptorsDiscoveryActive(const DiscoveredCharacteristic& characteristic) const;

virtual void terminateCharacteristicDescriptorsDiscovery(const DiscoveredCharacteristic& characteristic);

[rgrover]

Please add a doxygen header describing the purpose of this class. We'll need to do this for everything.

[rgrover]

I believe it should be isCharacteristicDescriptorDiscoveryActive(). Singular. Consistent with isServiceDiscoveryActive where we are also discovering a set of services.

[pan-]

Thanks I will update all signatures.

[pan-]

@rgrover, @andresag01 Could you review these changes ?

[andresag01]

+1