Make it easy to generate a new PYSEC advisory by hand

[di]

While most advisories are automatically generated, occasionally they will be hand-written (e.g. #72).

It would be nice to add a tool (even if it's just a bash script) that can generate a template file and choose the latest PYSEC ID number, set timestamps, etc for instances when these need to be done by hand.

[woodruffw]

For inspiration: I don't know how GitHub did it, but the UI for modifying GHSA entries is incredibly nice.

For example: https://github.com/advisories/GHSA-p6h9-gw49-rqm4/improve

(I looks Markdown driven, but I'm not sure if this is a public feature yet.)

[westonsteimel]

Yeah, I think the easiest path forward on that would be to get our automation capable of pulling in the updates from GHSA per google/osv.dev#254, then we could just point people there to submit the improvement and it'd feed in here automatically. Would only work for stuff that already has a GHSA id though since you can't currently create a new advisory that way

[oliverchang]

This is an area that we certainly need more investment in. Our OSV.dev team will likely be able to prioritize some work here to help with manual generation of entries and UX for that next year.