Add RichTextInputElement to slack_sdk.models

[seratch]

Summary

This pull request adds RichTextInputElement to slack_sdk.models utilities.

references:

• https://api.slack.com/reference/block-kit/block-elements#rich_text_input

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.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/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 python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[codecov]

Codecov Report

Merging #1406 (a353286) into main (398923a) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1406      +/-   ##
==========================================
+ Coverage   85.37%   85.39%   +0.02%     
==========================================
  Files         111      111              
  Lines       11860    11871      +11     
==========================================
+ Hits        10125    10137      +12     
+ Misses       1735     1734       -1     

Files	Coverage Δ
slack_sdk/models/blocks/__init__.py	100.00% <100.00%> (ø)
slack_sdk/models/blocks/block_elements.py	95.63% <100.00%> (+0.09%)	⬆️

... and 2 files with indirect coverage changes

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