Add support for private repositories

[jericop]

Describe the Enhancement

The current implementation of poetry-install does not support private repositories. This forces users to work around this limitation by downloading dependencies before a build and modify their pyproject.toml file to reference a local path as shown below.

[tool.poetry.dependencies]
python = ">=3.13, <4"
# Need to download some-package before the build because the buildpack does not support private repositories.
some-package = {path="./local-copy-of-some-package"}

Possible Solution

For private GitHub repos the git buildpack supports a git-credential binding that could be used.
This thread has some more information on implementation.
https://github.com/orgs/python-poetry/discussions/3794

Poetry provides general support for private repositories as documented here. The buildpack could be updated to make this possible.
https://python-poetry.org/docs/repositories/

Motivation

Not all apps are built with public dependencies, and the poetry-install buildpack should support private dependencies.