feat: add nix build definitions

[aws-nslick]

Issue #, if available:

Description of changes:

This adds a nix flake for aws-ofi-nccl.

Usage:

1. To use the development shell, install direnv then create .envrc with contents:

watch_file nix/shell.nix
use_flake . --impure

This will create a .clangd file which can be used for IDE-like features atop clangd. Individual editor configuration varies. Example emacs configuration using doom emacs:

(use-package! eglot
  :config
  (add-to-list 'eglot-server-programs
               `((c-mode c-ts-mode c++-mode c++-ts-mode objc-mode)
                 . ("clangd"
                    "-j=8"
                    "-background-index-priority=normal"
                    "--enable-config"
                    "--all-scopes-completion"
                    "--header-insertion=iwyu"
                    "--header-insertion-decorators"
                    "--log=verbose"
                    "--clang-tidy"
                    "--background-index"
                    "--malloc-trim"
                    "--completion-style=detailed"
                    "--import-insertions"
                    "--pch-storage=memory"
                    "--function-arg-placeholders=1"))))

2. To build aws-ofi-nccl, libfabric, openmpi, etc.: nix build .#default.

3. To build wrappers for nccl-tests that run atop ubuntu, run: nix build .#ubuntu-test-runners. ./result/bin/ will contain shell scripts that LD_PRELOAD nvidia libraries from ubuntu expected locations, then forwards args to the nccl-test in question.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[bwbarrett]

Nix isn't really my thing, but I have no objection to most of this patch (other than the Libfabric patch I called out below). Other than that, I'm not sure it's a competent review, but I'd be fine approving.

[aws-nslick]

@ConnorBaker if you have any free time, any comments or criticisms are much appreciated.