feat: add attrs/fields for the runtime's ABI flags

[rickeylev]

This adds attributes and fields for storing the runtimes ABI flags value, i.e. sys.abiflags.

For freethreaded interpreters, the abi flags contain t, which is used creating e.g.
virtualenvs.

The attribute can be directly specified, or if it's not, will be computed based on the
--py_freethreaded flag.

[aignas]

What about doing

diff --git a/python/private/hermetic_runtime_repo_setup.bzl b/python/private/hermetic_runtime_repo_setup.bzl
index 3f7bb5d7..207d021b 100644
--- a/python/private/hermetic_runtime_repo_setup.bzl
+++ b/python/private/hermetic_runtime_repo_setup.bzl
@@ -198,6 +198,10 @@ def define_hermetic_runtime_toolchain_impl(
             _IS_FREETHREADED: "cpython-{major}{minor}t".format(**version_dict),
             "//conditions:default": "cpython-{major}{minor}".format(**version_dict),
         }),
+        abi_info = select({
+            _IS_FREETHREADED: "t",
+            "//conditions:default": "",
+        }),
     )

     py_runtime_pair(

Instead of depending on the _py_freethreaded_flag implementation?