Generate CI configuration with maturin generate-ci

[messense]

Output CI config to stdout, with some options to control the output

• maturin generate-ci <CI>: CI provider, initially only github is supported
• --platform=linux windows macos emscripten: platforms to build, possibly also support something like linux/arm64 for arm64 Linux and linux/*(or just linux) for all supported Linux architectures
  • Add support for Emscripten in generate-ci command #1484
• --python-implementation=cpython,pypy: Control Python implementation support
• --pytest: Enable pytest
• Infer abi3 from cargo metadata like we already do in maturin build
• --zig: Prefer to cross compile with zig instead of docker containers
• Support bin, cffi, rust-cpython and uniffi bindings
  • Add bin bindings support to generate-ci command #1470
• Replace CI.yml template in new/init command: Replace CI.yml template with generate-ci #1472

This will make it easier for projects to switch from abi3 to non-abi3 or vice-versa, messing with CI is painful.

[messense]

• possibly also support something like linux/arm64 for arm64 Linux and linux/*(or just linux) for all supported Linux architectures
• --python-implementation=cpython,pypy: Control Python implementation support

We can add these in the future if users really want them.