feat: support for use through jvm -javaagent command line option

[jkorri]

I'd like to use the library for various testing scenarios so that I don't have to add a dependency to the project I'm testing, or try to manipulate a running jvm before dns dependent code gets run. This can be achieved by running the agent at JVM startup.

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 75.75%. Comparing base (72c8073) to head (390860e).

Files with missing lines	Patch %	Lines
.../src/main/java/com/alibaba/dcm/agent/DcmAgent.java	33.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #289      +/-   ##
============================================
+ Coverage     75.26%   75.75%   +0.48%     
- Complexity      162      164       +2     
============================================
  Files            11       11              
  Lines           651      664      +13     
  Branches         84       84              
============================================
+ Hits            490      503      +13     
  Misses          119      119              
  Partials         42       42              

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[oldratlee]

LGTM

---

@jkorri Thanks for your professional contributions, merged 💕

[oldratlee]

@jkorri released v1.8.3:
https://github.com/alibaba/java-dns-cache-manipulator/releases/tag/v1.8.3