Basic implementation for read blob, read dynamic attributes, prepare and execute write and find information

[bjoernQ]

This PR adds basic support for

• read blob
• reading dynamic attributes (i.e. dynamic length and content)
• prepare and execute write
• bare bones support for find information

I tested the the implementation on a NRF52832DK board, and it seems to work as expected. I didn't added this to the demo code contained in this repository since I guess it might cause too much confusion for someone just wanting to learn Rubble.

But I have some demo code at https://github.com/bjoernQ/rubble-no-rtic-demo/tree/demo_additional_requests

I am not really happy about the fixed size of the buffer for dynamic read attributes but I have no better idea for now.

I am not too sure if the way that find information is implemented is a bit too low level - but at least it gives the user ultimate choice about the implementation.

[jonas-schievink]

Cool! Haven't reviewed in depth but this seems good!