Add slack message adapter

[timkpaine]

Still needs a lot of work, just putting this draft PR up so its out there

• finish basic functionality
  • receive messages and map to SlackMessage struct
  • send messages, map from SlackMessage struct
  • handle tagged users
  • Map user and channel IDs to user and channel names
  • Pull user email from user id and make available on message
  • disambiguate public, private, and direct messages
  • Handle emoji reactions
• add unit tests with mocked out slack SDK
• add extras to pyproject.toml
• have subscribe return ts[[msg]] to match SymphonyAdapter

fixes: #17
ref: #35

MVP:

import csp
from csp import ts
from csp.adapters.slack import SlackAdapterManager, SlackMessage

SLACK_APP_TOKEN = "xapp-..."
SLACK_BOT_TOKEN = "xoxb-..."

@csp.node
def node(msg: ts[SlackMessage]) -> ts[SlackMessage]:
    if csp.ticked(msg):
        text = f"Hello <@{msg.user_id}>!"
        return SlackMessage(
            channel=msg.channel,
            channel_id=msg.channel_id,
            msg=text,
        )

def graph():
    am = SlackAdapterManager(SLACK_APP_TOKEN, SLACK_BOT_TOKEN)
    am.publish(node(am.subscribe()))

csp.run(graph, realtime=True)