Add Support for apps.manifest.* Endpoints

[misscoded]

Add support for the following apps.manifest.* methods:

apps.manifest.create
apps.manifest.validate
apps.manifest.update
apps.manifest.delete
apps.manifest.export

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

Requirements

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[avalatea]

Hey, just want to make sure I am interpreting things correctly. Are these the modules that still needs app manifest support before the manifest feature can be pushed out?

• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

It looks like the docs might have assumed that this functionality exists when it seems to still be a WIP.

[misscoded]

Fixed in #1123.