[SDK] Add delegate function to allow dynamic setting of headers for all requests from outside the SDK code

### Description

**What would you like to do?**
Whenever a request is made to the matrix server from the matrix SDK (e.g. send message) then would like a delegate method to be called (which can be hooked into from the app) and extra headers can be set.

**Why would you like to do it?**
Set extra request headers for security.

**How would you like to achieve it?**
In the app be able to hook into a "onRequestBuild" delegate function and define your own dynamic headers...
matrixSdk.onRequestBuild = (requestBuilder: Request.Builder) {
requestBuilder.header("My-Authorization", "Bearer ${token}")
requestBuilder.header("Another-One", "another value")
requestBuilder.header("Local-Time", Datetime.now())
}

**Have you considered any alternatives?**
Yes only alternative is to copy all the SDK code into the app then make the adjustments directly to the SDK.