Add fallback argument in BlockAttachment

[seratch]

Summary

The attachments document encourages having fallback field in each attachment: https://api.slack.com/reference/messaging/attachments#legacy_fields

This pull request resolves the missing fallback argument in BlockAttachment constructor.

Thanks @eddyg for the feedback: https://community.slack.com/archives/CHL4CLRCZ/p1616013860007200
(if you're interested in joining the Slack workspace, you can join the community from https://j.mp/community-slack-com)

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)
• /docs-src (Documents, have you run ./docs.sh?)
• /docs-src-v2 (Documents, have you run ./docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python setup.py validate after making the changes.

[codecov]

Codecov Report

Merging #979 (8c8ba34) into main (0ac8a0b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #979      +/-   ##
==========================================
+ Coverage   87.31%   87.34%   +0.02%     
==========================================
  Files          94       94              
  Lines        8541     8543       +2     
==========================================
+ Hits         7458     7462       +4     
+ Misses       1083     1081       -2     

Impacted Files	Coverage Δ
slack_sdk/models/attachments/__init__.py	99.41% <100.00%> (+<0.01%)	⬆️
slack_sdk/socket_mode/builtin/internals.py	72.72% <0.00%> (-0.44%)	⬇️
slack_sdk/socket_mode/client.py	76.99% <0.00%> (+2.65%)	⬆️

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[eddyg]

LGTM! 👍