feat: add admin.users.getExpiration method

[srtaalej]

This PR adds the admin.users.getExpiration method to the SDK.

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

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you agree to those rules.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.30%. Comparing base (bf988dc) to head (f60257e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...slack/api/methods/impl/AsyncMethodsClientImpl.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1543      +/-   ##
============================================
- Coverage     73.32%   73.30%   -0.02%     
- Complexity     4511     4513       +2     
============================================
  Files           477      477              
  Lines         14266    14274       +8     
  Branches       1487     1487              
============================================
+ Hits          10460    10464       +4     
- Misses         2916     2919       +3     
- Partials        890      891       +1     

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[zimeg]

@srtaalej LGTM! Thanks for such a fast follow up with more methods! 👾 ✨

[zimeg]

⭐ praise: It's great to find a user here!