Add confirm.style in blocks

[seratch]

Summary

This pull request adds style to confirm composition objects in Block Kit.
https://api.slack.com/reference/block-kit/composition-objects#confirm

Related issues/changes:

• Support 'style' on Confirm object node-slack-sdk#1022
• Add style to confirm in a block element python-slack-sdk#699

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

Merging #470 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #470      +/-   ##
============================================
- Coverage     83.65%   83.59%   -0.07%     
+ Complexity     2387     2386       -1     
============================================
  Files           250      250              
  Lines          6442     6442              
  Branches        590      590              
============================================
- Hits           5389     5385       -4     
- Misses          690      694       +4     
  Partials        363      363              

Impacted Files	Coverage Δ	Complexity Δ
...pi/methods/metrics/impl/RedisMetricsDatastore.java	87.03% <0.00%> (-2.47%)	37.00% <0.00%> (-1.00%)

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[stevengill]

LGTM