Add option to cargo vendor crates into sdist

[messense]

The maturin side of https://github.com/PyO3/setuptools-rust/issues/112

~~Currently maturin has no sdist command, it builds sdist along with binary wheels. Do we want a seperated sdist command and add vendoring support to that~~, or do we want to directly add vendoring support to existing build command?

[konstin]

I'm a bit wary not to grow too many cli options, but a --vendor flag sounds reasonable

[sushinoya]

Has there been any progress on this issue since 2021? Or alternatively, is there support within maturin now to specify a directory of vendor-ed crates to support offline builds?

[messense]

No progress yet since no one asked for it before.