execute fetchers as code action

[MangoIV]

Is your feature request related to a problem? Please describe.
often times, you have some fetcher and then you go back to the terminal to execute them to get the hash. It would be immensely useful to have that within nixd as a code action or code lense

Describe the solution you'd like
when using something like fetchFromGitHub, offer to augment the passed attribute set with the hash computed from prefetching it.

Describe alternatives you've considered
n/a

Additional context
Another options would be to have a code lens automatically populated whenever the language server detects that there can be soemthing prefetched, e.g.

• language server sees that there's a call to fetchFromGithub
• language server spawns thread that fetches this
• language server adds code lens that offers to add a hash = line to the passed attribute set.

[inclyc]

Duplicated to #461