Add "unstable" frame stack api

GitHub fields:

assignee = 'https://github.com/markshannon'
closed_at = None
created_at = <Date 2022-04-04.11:59:07.553>
labels = ['expert-C-API', 'type-feature', '3.11']
title = 'Add "unstable" frame stack api'
updated_at = <Date 2022-04-04.12:09:47.075>
user = 'https://github.com/markshannon'

bugs.python.org fields:

activity = <Date 2022-04-04.12:09:47.075>
actor = 'Mark.Shannon'
assignee = 'Mark.Shannon'
closed = False
closed_date = None
closer = None
components = ['C API']
creation = <Date 2022-04-04.11:59:07.553>
creator = 'Mark.Shannon'
dependencies = []
files = []
hgrepos = []
issue_num = 47215
keywords = ['patch']
message_count = 1.0
messages = ['416665']
nosy_count = 2.0
nosy_names = ['Mark.Shannon', 'brandtbucher']
pr_nums = ['32303']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue47215'
versions = ['Python 3.11']