Optimize the default prompt and make prompt customizable for create anomaly detector tool

[gaobinlong]

Description

Optimize the prompt feed to LLM when executing the CreateAnomalyDetectorTool, the change are:

1. Add (long, integer, double, float, short etc.).
2. Add you should give at most 3 aggregation fields and corresponding aggregation methods

, and also make the prompt customizable, users can use their customized prompt.

Related Issues

#337

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[gaobinlong]

@zane-neo @xinyual please help to review this PR, thank you!

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.65%. Comparing base (2b76e3c) to head (477ec5a).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
...nsearch/agent/tools/CreateAnomalyDetectorTool.java	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #399      +/-   ##
============================================
- Coverage     81.78%   77.65%   -4.14%     
- Complexity      193      260      +67     
============================================
  Files            11       15       +4     
  Lines           961     1383     +422     
  Branches        137      197      +60     
============================================
+ Hits            786     1074     +288     
- Misses          121      232     +111     
- Partials         54       77      +23     

Flag	Coverage Δ
	77.65% <66.66%> (?)

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[gaobinlong]

The test case ./gradlew ':integTest' --tests "org.opensearch.integTest.RAGToolIT.testRAGToolWithNeuralQuery" always fails on MacOS, but not relates to this PR.