[REST] make selection of output-format mandatory, support dot url syntax

1. Remove the default format (binary) because `rest/block/<hash>/Hex` would end up delivering binary data.
2. List available formats when chosen format was not found (reduces need for documentation)
3. Change url syntax to extension like format chosing (like `rest/tx/<hash>.json` (as originally mentioned in #2844)
4. Supports uppercase, camelcase format parameter like `.Json` or `.Hex/`