Introduce link to proposal in proposal submissions

[Valentin Trinqué]

Every proposal transaction contains the following common fields:

• a link to a text file in markdown format and
• a cryptographically secure hash (SHA3-512) of the text so that viewers can check that the text hasn't been changed since the proposal was submitted and
• a description field to show a short title / something in case the link goes offline. This is to be between 0 and 255 unicode characters.

Related the spec:

• vegaprotocol/specs#1006

Related issue:

• vegaprotocol/vega#5192