feat: add extra metadata files while building wheels

[shubhbapna]

fixes #319

We will create a BUILD file in the .dist-info directory of the wheel. This file is of TOML format and contains the entire settings of the package by default. There is an option for the user to include more data in this file by defining a plugin.

Additionally, I added the build system, build backend and build sdist requirements file (hope is that we can use this later to speedup bootstrapping)

[dhellmann]

Does this over-write the file, or append to it? Maybe what we really want to do is add the extra data from the plugin to the other settings as a key in the other dict. Something like custom-metadata-extensions? It's a long name, so something shorter that captures the meaning might be better.

Does TOML impose any restrictions on the syntax for keys? (no spaces, case-insensitive, etc.)

[shubhbapna]

okay I can either add the custom metadata in some key or another option is that i make the settings data as the default function. If a plugin is specified then all the data is added only from there?

TOML does have some restrictions: https://github.com/toml-lang/toml/blob/main/toml.md#keys

[dhellmann]

How likely is it that data we are pulling from YAML or other places won't be compatible with TOML?

[shubhbapna]

i think the tomlkit handles it for us. It was able to successfully convert this yaml without an error:

packages:
  stevedore:
    download_source:
      url: https://files.pythonhosted.org/packages/e7/c1/b210bf1071c96ecfcd24c2eeb4c828a2a24bf74b38af13896d02203b1eec/stevedore-${version}.tar.gz
      destination_filename: stevedore-custom-${version}.tar.gz
    resolver_dist:
      sdist_server_url: "https://pypi.org/simple"
      include_sdist: false
      include_wheels: true
    changelog:
      "1.0.0 hahaha":
        - test hello world