Expose features for some OT CMake settings

[ivmarkov]

We probably want to expose features in openthread-sys (re-exported by openthread) for the following settings:

            .define("OT_LOG_LEVEL", "NOTE")
            .define("OT_FTD", "OFF")
            .define("OT_MTD", "ON")
            .define("OT_RCP", "OFF")
            .define("OT_TCP", "OFF")
            .define("OT_APP_CLI", "OFF")
            .define("OT_APP_NCP", "OFF")
            .define("OT_APP_RCP", "OFF")
            .define("OT_BORDER_ROUTER", "OFF")
            .define("OT_BORDER_ROUTING", "OFF")
            .define("OT_SRP_CLIENT", "ON")
            .define("OT_SLAAC", "ON")
            .define("OT_ECDSA", "ON")
            .define("OT_PING_SENDER", "ON")

Combined with the existing force-generate-bindings feature, that would allow the user to generate their own sys crate, that suits their needs, rather than using the (possibly pre-built) defaults.

[bjoernQ]

I don't even know what OT_APP_* does but I guess when on it we can just expose all of them 👍