Add URI conversion facilities

[segfaultxavi]

Converters to transform from URI to Payload and viceversa would be useful.
In this way, the "How to sign transaction offline" guide would be much simpler and could be done completely using symbol-cli.

Something like:

$ symbol-cli converter payloadToURI
Enter payload: XXXXXXXXXXXXXXXXXXXXXX
web+symbol://transaction?data=XXXXXXX&generationHash=XXXXXXX&nodeUrl=XXXXXX&webhookUrl=XXXXXX

This URI can then be used with symbol-cli transaction uri.

Bonus points if symbol-cli transaction could directly accept payloads instead of URIs :)