Add `pgo` config to enable PGO on builds

[winstxnhdw]

With PGO builds introduced in 1.13, could we also have a pgo config?

[tool.maturin]
pgo = true
pgo-command = "python workload.py"

I would like to enable PGO builds on pip install without having to explicitly pass the --pgo flag.

[davidhewitt]

FWIW I find that not all build environments support pgo well, so this will probably need to be overridable from the command line with --no-pgo

[messense]

I think it's cleaner to keep the current opt-in default, passing an extra --pgo argument isn't that hard.

[winstxnhdw]

With pip, it is. You have to set an env var instead..