Expose `sysconfig` variables in `PyRuntimeInfo` provider

# 🚀 feature request

Downstream rules are potentially interested in the selected toolchains configuration.
Specifically, building C extension modules depends on things like `EXT_SUFFIX` to create proper extensions supporting the chosen interpreter.
Because that suffix ends up in file names, downstream rules may need that value at configure time, when they can't query the interpreter.
But the `@rules_python` `repository_rule` can easily determine these variables before configure time, for each registered toolchain.

This is related to:
- https://github.com/bazelbuild/rules_python/issues/824
- https://github.com/nicholasjng/nanobind-bazel/issues/61
- I'm sure other downstream bazel tooling related to C extensions would like this as-well

### Relevant Rules

This feature would ultimately need changes to:
- `PyRuntimeInfo` (basic schema)
- `py_runtime` (so that third party toolchains/interpreters can provide that information too)
- the internal pluming providing the `@rules_python` provided internal toolchains

### Description

`PyRuntimeInfo` needs a field `ext_suffix`, or alternatively `sysconfig_variables` (for future extension).
At extension instantiation time, when setting up the toolchain repos, the respective interpreters should be queried for their `ext_suffix`; alternatively, that information could be determined offline.

Start with the PyRuntimeInfo schema and the py_runtime rule named in the issue, then trace the internal toolchain repository setup that supplies registered interpreters. Review issues #824 and nanobind-bazel#61 for context; done means sysconfig data such as ext_suffix is available to downstream rules and can be provided by third-party toolchains.

Written by the indexing model from the issue text.