feat: add possibility to set the pyproject TOML on a Poetry instance

[DavidVujic]

Resolves part one of: python-poetry#7039

• Added tests for changed code.
• Updated documentation for changed code.

[sonarcloud]

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0.0% Duplication

[neersighted]

I think this is an inferior duplicate of python-poetry/poetry#6810 (it's not necessary to add special support for overriding the project file as there are other places in the code that assume its location at the root of the project, and we can set CWD instead as in that PR).

[DavidVujic]

I think this is an inferior duplicate of python-poetry/poetry#6810 (it's not necessary to add special support for overriding the project file as there are other places in the code that assume its location at the root of the project, and we can set CWD instead as in that PR).

Thank you for explaining and linking to the already ongoing work. I'll close this one!