Update channel settings

Partial update of a channel's settings (send only the keys you want to change). Keys are per channel type. Blooio (P2P numbers): auto_mark_read is available on dedicated and inbound lines; auto_share_contact is dedicated-only AND the line must have a contact card (name or photo) set up; imessage_only is available on every line. Apple Messages for Business and WhatsApp Business: opener, a partial update of the channel's opener (send only the fields you want to change). Turning a setting OFF is always allowed. Errors return a machine-readable code: settings_unsupported_for_channel_type (403, a channel type without settings), auto_mark_read_not_allowed / auto_share_contact_not_allowed (403, line type can't enable it), contact_card_not_set_up (403, enable auto_share_contact with no contact card), no_settings_provided (422, empty update), invalid_settings (422, unknown key or field, a value of the wrong type, or an opener turned on without its template), invalid_invitation (422, an AMB opener template missing a required parameter), template_not_approved (422, a WhatsApp opener template that isn't approved on this channel). A malformed channel selector or non-object body is 400.