[slack] Update resource endpoints

[animeshk08]

This commit updates the resource endpoints
to fetch the messages and information of a
channel.
The tests have been updated accordingly.

Signed-off-by: Animesh Kumar animuz111@gmail.com

[animeshk08]

The following are the changes due to the new of endpoints:

• Additions to channel_info:

"is_ext_shared": false,
"is_group": false,
"is_im": false,
"is_pending_ext_shared": false,
"parent_conversation": null,
"pending_connected_team_ids": [],
"pending_shared": [],
"shared_team_ids": [
                "T01281XCKTR"
            ],

• Deletions from channel_info:

"latest": {
                "blocks": [
                    {
                        "block_id": "WoeZ4",
                        "elements": [
                            {
                                "elements": [
                                    {
                                        "text": "hello",
                                        "type": "text"
                                    }
                                ],
                                "type": "rich_text_section"
                            }
                        ],
                        "type": "rich_text"
                    }
                ],
                "client_msg_id": "0c9d5905-c9fc-4f71-a067-b48fe248ebee",
                "parent_user_id": "U01289NA1HV",
                "team": "T01281XCKTR",
                "text": "hello",
                "thread_ts": "1588129375.001100",
                "ts": "1588132356.001800",
                "type": "message",
                "user": "U01289NA1HV"
            },
            "members": [
                "U012UUR4V6Y",
                "U01289NA1HV",
                "U012MC9PEER"
            ]

• I have manually tried the slack implementation on various kinds of messages and did not find any change in the history API response.

Based on the current implementation of the enricher for slack I don't think any extra fields are to be enriched(please let me know if any of the above addition is to be enriched) only the latest may be removed from the raw index mapping of slack.