Add functionality for third-party parameter sets

[agriyakhetarpal]

This PR adds: a cookiecutter.json configuration to ask users if they wish to register a third-party parameter set, a basic example of a parameter set, and an entry point in pyproject.toml for using it with PyBaMM

[agriyakhetarpal]

I shall be adding documentation and tests for the same in further pull requests, but I tested this with an example project and it works as intended (the parameter set is defined in src/pybamm_cookiecutter/my_parameters.py).

in pyproject.toml

[project.entry-points.pybamm_parameter_sets]
custom_parameters = "src.pybamm_cookiecutter.my_parameters:get_parameter_values"

and testing it as follows:

>>> pybamm.parameter_sets.get_docstring("custom_parameters")

'\nExample docstring for a third-party parameter set titled `my_parameters`.\n\nReferences\n----------\n\nAdd references to the relevant papers here in this format, to be parsed by\nsphinxcontrib.bibtex in the documentation through a CITATIONS.bib file. You\nmay refer to the documentation for this extension at\nhttps://sphinxcontrib-bibtex.readthedocs.io/en/latest/\n\n:footcite:`van1995python`\n\n'

[Saransh-cpp]

Looks good, thanks, @agriyakhetarpal!

[valentinsulzer]

could you add an example function and scalar parameter?

[agriyakhetarpal]

I added a sample function with scalar parameters from the OKane2022 parameter set, let me know if that suffices

[agriyakhetarpal]

I will close this to allow @santacodes to work on this further. He will be looking to start with setting up some testing infrastructure and CI/CD, some additional build backends, and he will then proceed to add some parameter sets and a data file plugin (subject to further discussion on the design for the latter).