feat: support arbitrary path exports

[ajaxbits]

This will allow the user to specify arbitrary paths in the pyproject.toml file for the action to export in its project json object.

For example, if the pyproject.toml looks like:

[tool.poetry]
name = "single-project"
version = "0.1.0"
description = ""
authors = ["Your Name <you@example.com>"]
readme = "README.md"

[tool.poetry.dependencies]
python = "^3.9"

[tool.export.me.please]
foo = "bar"
true = true
one = 1


[tool.poetry.group.dev.dependencies]
pytest = "^7.4.0"

[build-system]
requires = ["poetry-core"]
build-backend = "poetry.core.masonry.api"

And the user specified tool.export.me.please as the value for additional-export-paths, then the projects output for that project will look like:

{
  "buildBackend": "poetry.core.masonry.api",
  "commands": {
    "install": "poetry install",
  },
  "directory": "test-fixtures/project-with-exports",
  "name": "single-project",
  "path": "test-fixtures/project-with-exports/pyproject.toml",
  "pythonVersion": "^3.9",
  "exports": {
    "tool.export.me.please": {
      "foo": "bar",
      "one": 1,
      "true": true,
    }
  }
}

[BrandonLWhite]

Thanks for the contribution! This will be a very handy feature.