C API: Flakes, locking, override-input

[roberth]

Is your feature request related to a problem? Please describe.

Currently flakes are accessible via builtins.getFlake (when the flag is enabled of course), but functionality that is expected from any flakes-supporting CLI will include at least:

• --override-input and
• automatically updating the lock file despite pure mode.

Describe the solution you'd like

At least one of

• A CLI oriented helper struct that performs these tasks and returns a struct with access to the outputs value and/or eval cache
• A perhaps slightly more flexible/low level set of functions that manipulate the lock and can evaluate a flake with it
• Both
  • a C function that returns a "locked" flake regardless of pure mode.
  • Nix-language support for overriding flake inputs

Additional context

#8699

Priorities

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