[REST] make selection of output-format mandatory, support dot url syntax
[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 dot extension like format chosing (like `rest/tx/<hash>.json`

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