Add descriptor and attribute value to gatt_service proc macro

[HaoboGu]

Descriptor and attribute value settings are commonly used in BLE characteristic, with this PR, they can be specified as:

#[nrf_softdevice::gatt_service(uuid = "1812")]
pub struct HidService {
    #[characteristic(
        uuid = "2A4D",
        read,
        write,
        value = "[0u8, 1u8]",
        descriptor(uuid = "2908", security = "justworks", value = "[0, 1]")
    )]
    pub data: [u8; 1],

[alexmoon]

Can you add an example to the examples project demonstrating how this is used?

[HaoboGu]

Sure, will do!

[alexmoon]

Is it possible to allow multiple descriptors per characteristic by repeating the descriptor(...) argument?

[HaoboGu]

It might be possible, I'm not sure.. will have a try tomorrow!

[HaoboGu]

Multiple descriptor fields is supported. Just tested it, works fine.