Add JSON schema generation for pyproject.toml with schemars for SchemaStore

[JonathanPlasse]

Adding an automatic JSON schema generation of the maturin configuration in pyproject.toml would allow update SchemaStore easily and enable autocompletion and validation in pyproject.toml.
Ruff could be taken as an example, as it has an automatic JSON schema generation that use schemars.
Schemars only need to add some macro on the configuration struct.
The generation would be available through a feature.
I am willing and able to implement it.

This feature has been requested on the SchemaStore repo.

• Missing PyProject schemas SchemaStore/schemastore#3564

[messense]

Thanks, pull requests are welcome!