Add expand prfoperty to section block type

[seratch]

Summary

This pull request adds a missing property to section block type: https://api.slack.com/reference/block-kit/blocks#section_fields

see also:

• Fix #1611 Add expand attribute to SectionBlock python-slack-sdk#1635
• Add #1416 Add expand property to SectionBlock class java-slack-sdk#1420

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.89%. Comparing base (6c29b1b) to head (6b52023).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2137   +/-   ##
=======================================
  Coverage   91.89%   91.89%           
=======================================
  Files          38       38           
  Lines       10320    10320           
  Branches      649      649           
=======================================
  Hits         9484     9484           
  Misses        824      824           
  Partials       12       12           

Flag	Coverage Δ
cli-hooks	95.24% <ø> (ø)
cli-test	94.48% <ø> (ø)
oauth	77.39% <ø> (ø)
socket-mode	61.82% <ø> (ø)
web-api	96.88% <ø> (ø)
webhook	96.65% <ø> (ø)

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