Allow overriding underlying buildRustCrate invocation

[kolloch]

I want to make it easy to build a crate with default features etc but also to customize it. So far, I have allowed this by providing .build attribute which contains the derivation with the default arguments. That derivation is returned by buildRustCrateWithFeatures which uses makeOverridable to allow overriding its paramters.

Unfortunately, buildRustCrate in nixpkgs also uses makeOverridable to allow customization. Sometimes, it might be good to customize the derivation on that level.

I can think of multiple ways to achieve this:

1. Instead or in addition of the build attribute, I would introduce a builder attribute which is a function that you can call with an empty attr set {} to get the old behavior -- or specify additional parameters to override behavior.
2. Add an overridable buildRustCrateOverride parameter to buildRustCrateWithFeatures. That parameter would contain a function which is passed to override on the buildRustCrate return value. E.g.:

   {/*...*/}.build.override {
       buildRustCrateOverride = (oldAttrs: {/*...*/});
   }

3. Maybe in addtion to 2 have a convenience .overrideBuildRustCrate function which crates an override on buildRustCrate using 2 or similar.

Tell me if that sounds like a good plan.