Add custom type mapping support

[bojankoma]

Complex objects need to be mapped to supported types in generator task and processor.
Add an extension map as seen in Apollo GraphQL for Android.

[bojankoma]

Similar to what we want to achieve is here.
Also, needs to be propagated to Collar core and processor like here.

[bojankoma]

After careful deliberation, I've decided to drop support for this feature.
Supported types are:

• java.lang.String
• boolean
• byte
• char
• double
• float
• int
• long
• short
• android.os.Bundle

If a developer wants to send over a more complex struct, for example a custom class, it needs to be serialized into a String and as such, it is the responsibility and final choice of the developer implementing Collar into its own project how to proceed.