Add convenient extensions for for writing and reading/observing bluetooth characteristic

[dasdranagon]

Is your feature request related to a problem? Please describe.
I'm always frustrated when need to make additional data transformations if I want to simply read/write/observe bluetooth characteristics.

Describe the solution you'd like
I would like to have an extension of device flow which allows to access flow of bytes arrays or discovered characteristic for writing/reading.

For example:

operator fun Flow<List<Service>>.get(uuidString: String) = this[uuidFrom(uuidString)] // Accss service with (string) id (so it can be used cross-platform)
operator fun Flow<List<Characteristic>>.get(uuidString: String) = this[uuidFrom(uuidString)] // Access characteristic with (string) id

fun Flow<Device?>.dataFlow(serviceUUID: String, characteristicUUID: String) = characteristicsFlow(serviceUUID, characteristicUUID).flatMapConcat {
    it.enableNotification()
    it
}.map { it ?: bytesOf() }

fun Flow<Device?>.characteristicsFlow(serviceUUID: String, characteristicUUID: String) = services()[serviceUUID]
    .characteristics()[characteristicUUID]
    .filterNotNull()

Usage:

deviceFlow.characteristicsFlow(SERVICE_UUID, RX_CHARACTERISTIC_UUID)
val data = deviceFlow.dataFlow(SERVICE_UUID, TX_CHARACTERISTIC_UUID).take(10).toList()

[dasdranagon]

Suggested by @thoutbeckers in https://github.com/splendo/health-sensors/pull/677/files