Close HostnameCache#executorService on SentryClient#close.

[maciejwalkowiak]

Fixes #1756.

📜 Description

Close HostnameCache#executorService on SentryClient#close.

💡 Motivation and Context

Fixes #1756.

💚 How did you test it?

Unit tests.

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

[marandaneto]

LGTM

[codecov-commenter]

Codecov Report

Merging #1757 (5fb81e9) into main (5cf90ef) will decrease coverage by 0.03%.
The diff coverage is 63.15%.

@@             Coverage Diff              @@
##               main    #1757      +/-   ##
============================================
- Coverage     75.34%   75.31%   -0.04%     
- Complexity     2158     2165       +7     
============================================
  Files           215      215              
  Lines          7699     7718      +19     
  Branches        820      822       +2     
============================================
+ Hits           5801     5813      +12     
- Misses         1502     1507       +5     
- Partials        396      398       +2     

Impacted Files	Coverage Δ
sentry/src/main/java/io/sentry/SentryClient.java	85.98% <55.55%> (-0.90%)	⬇️
...ry/src/main/java/io/sentry/MainEventProcessor.java	82.40% <57.14%> (-1.50%)	⬇️
sentry/src/main/java/io/sentry/HostnameCache.java	90.00% <100.00%> (+0.81%)	⬆️

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