Add default_value and descriptor to characteristic macro

[HaoboGu]

Current gatt service macro fills 0s as the AttributeData:

trouble/host-macros/src/service.rs

Line 149 in 224f371

let store = #name_screaming.init([0; size_of::<#ty>()]);

It would be very useful if users could set the value of AttributeData.

I found the following code blocks in host-macro, it seems value and descriptor macro args are not implemented. May I ask that is there any plan to finish this?

trouble/host-macros/src/characteristic.rs

Lines 115 to 126 in 224f371

	"value" => {
	return Err(Error::custom("Default value is currently unsupported").with_span(&meta.path.span()).into())
	// let value = meta
	// .value()
	// .map_err(|_| Error::custom("value must be followed by '= [data]'. i.e. value = 'hello'".to_string()))?;
	// default_value = Some(value.parse()?);
	},
	other => return Err(
	meta.error(
	format!(
	"Unsupported characteristic property: '{other}'.\nSupported properties are: uuid, read, write, write_without_response, notify, indicate, value"
	))),

[jamessizeland]

It is on the to-do list yes 😊, you're welcome to PR an implementation, but also good to know it's a wanted feature so we can prioritise it.

[HaoboGu]

yeah, this is commonly used in HID over BLE, for example, setting hid info and report map