Fix #1060 Enable Slack#close() method to shutdown thread pools behind its SlackConfig

[seratch]

This pull request resolves #1060 by enhancing Slack and SlackConfig classes to shutdown thread pools once close() method is called. Also, I've updated the javadoc of the SlackConfig class to clearly mention that reusing the instance is recommended.

To reviewers:

Please don't feel pressured even if you are not familiar with the internals of this SDK. To learn how SlackConfig and its sub config classes are connected to the ThreadPools classes, you can check how ThreadPools.getOrCreate() method works.

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.

[seratch]

The CI builds seem to be detecting some race condition issues:

closeMethod(test_locally.SlackConfigTest)  Time elapsed: 0.017 sec  <<< FAILURE!
java.lang.AssertionError: 
Expected: is <true>
     but: was <false>
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:8)
	at test_locally.SlackConfigTest.closeMethod(SlackConfigTest.java:167)

[seratch]

^ the above issue was due to the test code quality. improved.

[codecov]

Codecov Report

Merging #1064 (860be8a) into main (00c916e) will increase coverage by 0.01%.
The diff coverage is 81.48%.

@@             Coverage Diff              @@
##               main    #1064      +/-   ##
============================================
+ Coverage     76.78%   76.80%   +0.01%     
- Complexity     3718     3733      +15     
============================================
  Files           407      407              
  Lines         11197    11250      +53     
  Branches       1112     1128      +16     
============================================
+ Hits           8598     8640      +42     
- Misses         1928     1936       +8     
- Partials        671      674       +3     

Impacted Files	Coverage Δ
...n/java/com/slack/api/methods/impl/ThreadPools.java	54.54% <46.66%> (-6.57%)	⬇️
...-api-client/src/main/java/com/slack/api/Slack.java	73.46% <50.00%> (-0.84%)	⬇️
...lient/src/main/java/com/slack/api/SlackConfig.java	38.66% <100.00%> (+2.11%)	⬆️
...ain/java/com/slack/api/audit/impl/ThreadPools.java	100.00% <100.00%> (ø)
...main/java/com/slack/api/scim/impl/ThreadPools.java	100.00% <100.00%> (ø)
...va/com/slack/api/socket_mode/SocketModeClient.java	63.69% <0.00%> (-0.64%)	⬇️

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[WilliamBergamin]

Nice use of AutoCloseable, this looks good to me 💯

[filmaj]

LGTM! I today learned about AutoCloseable 🙇

[filmaj]

I just realized that these docs are hosted on sonatype.org, and as far as I can tell, can't be previewed locally in Jekyll 😢 .
I wanted to check to see if the references to various classes (i.e. SlackConfig) and maybe even properties (i.e. statsEnabled) would link properly. I suppose we won't be able to tell until we merge + deploy?

[seratch]

FYI: You can package javadoc.jar files by running mvn command and can unzip the file to check the generated HTML!