Add --platform option to facilitate installing wheels for platforms other than the host system

[BrandonLWhite]

The motivation for this PR is to add a new flag --platform that achieves the same effect as the pip option of the same name which is to facilitate the installation of wheels that are compatible with the platform specified.

The primary use case is in CI/CD operations to produce a deployable asset, such as a ZIP file for AWS Lambda and other such cloud providers. It is common for the runtimes of these target environments to be different enough from the CI/CD's runner host such that the binary wheels selected using the host's criteria are not compatible with the target system's.

Notes:

• This is not an actual cross-compiler. It simply allows controlling which prebuilt binaries are selected.
• I am committed to adding further documentation to the README for this setting if the project maintainers are open to accepting this feature.
• The env._supported_tags assignment that you will see in this code is not ideal, and I welcome any input for a better way. Since the Poetry code does not provide a public method for mutating this value, I resorted to mutating this pseudo-private field.
• I tried to use as much as I could from the packaging module, but I did have to reinvent the wheel for some things due to that module being hard-coded to call out to the host system to query certain values, which we are specifically trying to avoid here. In my research, I noted that the pip code effectively does the same thing. Should such a reusable module exist for producing the Tags combinations that are compatible with the specified platform, it would be much better to use that. I was not successful in finding such a library.