Add BLE UUID helper constants

[jamessizeland]

https://github.com/embassy-rs/trouble/issues/145 Follow this issue, to start adding UUID helper constants and constant functions to be exported from this repo.

Follow the spec (i.e. https://www.bluetooth.com/wp-content/uploads/Files/Specification/Assigned_Numbers.html#bookmark49)

Example pattern:

pub mod appearance {
    //! The representation of the external appearance of the device.
    //!
    //! This is a list of some of the most common appearance values and demonstrates the pattern to use to define new appearance values.
    //!
    //! https://www.bluetooth.com/wp-content/uploads/Files/Specification/Assigned_Numbers.html#bookmark49

    /// Construct a new appearance value for the GAP Service.
    ///
    /// Follow the pattern of the examples below to create new appearance values.
    /// Use UUIDs from the [Bluetooth Assigned Numbers list](https://www.bluetooth.com/wp-content/uploads/Files/Specification/Assigned_Numbers.html#bookmark49).
    ///
    /// ## Example
    ///
    /// ```rust
    /// use trouble_host::prelude::*;
    ///
    /// const GAMEPAD: &[u8; 2] = &appearance::new(0x00F, 0x040);
    /// ```
    pub const fn new(category: u8, subcategory: u8) -> [u8; 2] {
        (((category as u16) << 6) | (subcategory as u16)).to_le_bytes()
    }

    /// Generic Unknown device appearance.
    pub const GENERIC_UNKNOWN: [u8; 2] = new(0x000, 0x000);
    /// Generic Phone device appearance.
    pub const GENERIC_PHONE: [u8; 2] = new(0x001, 0x000);
    /// Generic Computer device appearance.
    pub const GENERIC_COMPUTER: [u8; 2] = new(0x002, 0x000);
    /// Smart Watch device appearance.
    pub const SMART_WATCH: [u8; 2] = new(0x003, 0x020);
    /// Generic Power device appearance.
    pub const GENERIC_POWER: [u8; 2] = new(0x01E, 0x000);
    /// Generic Sensor device appearance.
    pub const GENERIC_SENSOR: [u8; 2] = new(0x015, 0x000);
    /// Gamepad device appearance.
    pub const GAMEPAD: [u8; 2] = new(0x00F, 0x040);
}

[alexmoon]

Thinking about this more, we should probably make this a separate crate in the bt-hci workspace. We should auto-generate enums/consts from the Bluetooth SIG data available at https://bitbucket.org/bluetooth-SIG/public/src/main/ (probably include that repo as a submodule).

[jamessizeland]

Do you think we should re-export some bt-hci bits from trouble-host::prelude? Such as these UUIDs, Controller and ExternalController<>

[alexmoon]

I think that would be a perfectly reasonable thing for trouble to do if there are types/constants that apps need to use when interfacing with the host.