Discover ASDF interpreters by default (#20105)

I struggled for a while to debug the error messages I was getting from
pants (via pex), but it was ultimately because I had not added `<ASDF>`
to the search-path.

`["<PYENV>", "<PATH>"]` only puts the asdf shims on the path, and they
are powerless without the `.tool-versions`. It took me a long while to
understand what was happening because even in the sandbox it _looked_
like the asdf pythons should be findable.

In the spirit of making pants "just work" more often, and given the fair
popularity of asdf, should we support asdf by default?

---------

Co-authored-by: Benjy Weinberger <[email protected]>
Co-authored-by: Andreas Stenius <[email protected]>

src/python/pants/core/subsystems/python_bootstrap.py

@@ -89,7 +89,7 @@ class PythonBootstrapSubsystem(Subsystem):
 
     class EnvironmentAware(Subsystem.EnvironmentAware):
         search_path = StrListOption(
-            default=["<PYENV>", "<PATH>"],
+            default=["<PYENV>", AsdfPathString.STANDARD.value, "<PATH>"],
             help=softwrap(
                 f"""
                 A list of paths to search for Python interpreters.