Enrich nix flake init with parameters

[srid]

Enrich flake templates with a params attribute (eventually, these params will be upstreamed to the respective repos) that defines what to replace in the generated project directory post nix flake init.

This is meant to be consumed by https://github.com/juspay/flakreate which program does the following:

• Reads a registry of flake templates (by default, this repo)
• Prompt the user to pick a template
• Run nix flake init on it
• Prompt the user to choose replacement values (per the params spec, as you can see defined in this PR)
• Do the appropriate replacements.

Here's what it looks like to initialize https://github.com/juspay/nix-dev-home whilst doing the appropriate replacements (user name, email):

https://asciinema.org/a/rCOqlKyA3Og8b5WalKV9d1k97

To try this out locally,

nix run github:juspay/flakreate

[srid]

I'll merge this now, but we can continue the conversation for what the final params spec should be, in #7