Add thread_broadcast to MessageEvent subtype

[fussybeaver]

I received this payload on our prod

{"envelope_id":"becda5ef-218c-451c-ac02-d7176d310efb","payload":{"token":"hRcCJba8Je3CQu4LmsDNH5Ww","team_id":"T02AXGU3Z","api_app_id":"A02RNKPNM0S","event":{"type":"message","subtype":"thread_broadcast","text":"I\\u2019ll rollback anyway. What\\u2019s the procedure to rollback now that _redacted_ is fubared?","user":"U01QMNRB16G","ts":"1644918787.343029","thread_ts":"1644918662.376269","root":{"client_msg_id":"9978f034-7622-4548-ae46-c41bd1a5a5db","type":"message","text":"I just deployed _redacted_ :point_up::skin-tone-4:","user":"U01QMNRB16G","ts":"1644918662.376269","team":"T02AXGU3Z","blocks":[{"type":"rich_text","block_id":"yRCm9","elements":[{"type":"rich_text_section","elements":[{"type":"text","text":"I just deployed _redacted_ "},{"type":"emoji","name":"point_up","skin_tone":4}]}]}],"thread_ts":"1644918662.376269","reply_count":2,"reply_users_count":1,"latest_reply":"1644918787.343029","reply_users":["U01QMNRB16G"],"is_locked":false},"blocks":[{"type":"rich_text","block_id":"IBJ","elements":[{"type":"rich_text_section","elements":[{"type":"text","text":"I\\u2019ll rollback anyway. What\\u2019s the procedure to rollback now that _redacted_ is fubared?"}]}]}],"client_msg_id":"7df91135-f32a-4d70-86e1-88e5ad2b4cea","channel":"C06J3S97D","event_ts":"1644918787.343029","channel_type":"channel"},"type":"event_callback","event_id":"Ev033TS20WAU","event_time":1644918787,"authorizations":[{"enterprise_id":null,"team_id":"T02AXGU3Z","user_id":"U02SNDCEZB5","is_bot":true,"is_enterprise_install":false}],"is_ext_shared_channel":false,"event_context":"4-eyJldCI6Im1lc3NhZ2UiLCJ0aWQiOiJUMDJBWEdVM1oiLCJhaWQiOiJBMDJSTktQTk0wUyIsImNpZCI6IkMwNkozUzk3RCJ9"},"type":"events_api","accepts_response_payload":false,"retry_attempt":1,"retry_reason":"timeout"}

With this error:

    json_error: Error("unknown variant `thread_broadcast`, expected one of `bot_message`, `me_message`, `channel_join`, `bot_add`, `bot_remove`, `channel_topic`, `channel_purpose`, `channel_name`, `message_changed`, `message_deleted`, `tombstone`, `joiner_notification`, `slackbot_response`", line: 0, column: 0),

This patch attempts to fix this

[abdolence]

LGTM, thanks for the fix!