feat(flakes): `ref` field

[Malix-Labs]

Context

Flakes that are not of git type doesn't support targeting a different ref than the default ref (default branch)

Problem

Flakes that are not of git type and doesn't target the default ref (default branch) are forced to use the git field and append "?ref=" to their url string value to specify their (non default) ref

Solution

For flakes, feature a new "ref" field to specify for a non-default ref (different than the default branch)

Example

nixos-search/flakes/manual.toml

Lines 13 to 15 in 4e41fa5

	[[sources]]
	type = "git"
	url = "git+https://github.com/hannesgith/rquickshare?ref=flake"

With this solution, would become

[[sources]] 
type = "github"
owner = "hannesgith"
repo = "rquickshare"
ref = "flake"

For more, see #996

References

• nixos-search/flakes/manual.toml

  Lines 13 to 15 in 4e41fa5

  	[[sources]]
  	type = "git"
  	url = "git+https://github.com/hannesgith/rquickshare?ref=flake"

• nixos-search/flakes/manual.toml

  Lines 17 to 19 in 4e41fa5

  	[[sources]]
  	type = "git"
  	url = "git+https://github.com/hannesgith/neohtop?ref=nixify"

• nixos-search/flakes/manual.toml

  Lines 21 to 23 in 4e41fa5

  	[[sources]]
  	type = "git"
  	url = "git+https://github.com/raboof/octopin?ref=nix"

• nixos-search/flakes/manual.toml

  Lines 25 to 27 in 4e41fa5

  	[[sources]]
  	type = "git"
  	url = "git+https://github.com/TamtamHero/fw-fanctrl?ref=packaging/nix"

[raboof]

https://nix.dev/manual/nix/2.31/command-ref/new-cli/nix3-flake.html#flake-reference-attributes says ref is available on 'several' reference types, but doesn't say whether github is one of those. Is it? If so I think this feature would make sense.

[Malix-Labs]

Is it?

Yes for github, I'm using it in my local flake right now

I think all of the providers also does but I'm not sure