feat: add Nix flake and direnv support (#26005)

## Which issue does this PR close?

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Happy to file an issue if needed!

## Rationale for this change

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DataFusion requires
[protobuf](https://datafusion.apache.org/contributor-guide/development_environment.html#protoc-installation)
and OpenSSL installations to compile. Existing instructions only cover
[FHS](https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard)-compliant
systems.

## What changes are included in this PR?

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- Nix flake with protobuf, openssl, and pkg-config to avoid configuring
the paths manually
- Nix flake lockfile
- integration with https://github.com/nix-community/nix-direnv to
automatically load the flake when entering source code directory

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I have confirmed DataFusion now successfully compiles without having
protobuf or OpenSSL installed globally. Would appreciate confirmation
that this also works for other Nix[OS] developers!

## Are there any user-facing changes?

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No.

## Other considerations

I chose to track the stable
[channel](https://nixos.wiki/wiki/Nix_channels) to minimize the risk of
flake breaking. I chose to check-in the flake lock for reproducibility -
note that this means it will need to be periodically bumped.

This flake could be further extend to include other "supporting"
packages that DataFusion relies on for development, such as `uv` --
omitted those for simplicity of the PR / to not have to decide what goes
into flake VS not _now_.

I appreciate this is yet another developer-facing file that needs to be
maintained, and the project already has a devcontainer setup. However,
this provides a quick start for all developers using Nix[OS], without
forcing them to accept overhead of a Docker container, or figuring out
the native libraries installation themselves.

If nothing else -- every time I `git clone` a project that has flake +
direnv integration and automatically sets up a [Nix
shell](https://nixos.wiki/wiki/Development_environment_with_nix-shell)
that "just works", it makes my day a little bit and makes me that much
more likely to consider contributing. :) I want the same experience for
all (future) DataFusion contributors out there!

Author: VVKot

.envrc

@@ -0,0 +1 @@
+use flake

.gitignore

@@ -69,6 +69,10 @@ datafusion/core/*.parquet
 # Generated core benchmark data
 datafusion/core/benches/data/*
 
+# Generated Nix environment hooks
+
+.direnv/
+
 # rat
 filtered_rat.txt
 rat.txt

flake.lock

@@ -0,0 +1,28 @@
+{
+  description = "DataFusion development environment";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-26.05";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = {
+    nixpkgs,
+    flake-utils,
+    ...
+  }:
+    flake-utils.lib.eachDefaultSystem (system: let
+      pkgs = import nixpkgs {inherit system;};
+    in {
+      devShells.default = pkgs.mkShell {
+        nativeBuildInputs = [
+          pkgs.pkg-config
+        ];
+
+        buildInputs = [
+          pkgs.openssl
+          pkgs.protobuf
+        ];
+      };
+    });
+}