Expose `worker_target` and `workitem_cls` as arugments to customize `ThreadPoolExecutor` behaviour

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-09-06.00:14:20.180>
labels = ['3.7', 'type-feature', 'library']
title = 'Expose `worker_target` and `workitem_cls` as arugments to customize `ThreadPoolExecutor` behaviour'
updated_at = <Date 2022-01-24.22:42:12.007>
user = 'https://github.com/justdoit0823'

bugs.python.org fields:

activity = <Date 2022-01-24.22:42:12.007>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-09-06.00:14:20.180>
creator = 'justdoit0823'
dependencies = []
files = []
hgrepos = []
issue_num = 31357
keywords = []
message_count = 1.0
messages = ['301414']
nosy_count = 1.0
nosy_names = ['justdoit0823']
pr_nums = ['3268']
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31357'
versions = ['Python 3.7']