feat(java): add hardcoded secret rule (CWE-798)

[elsapet]

Description

Add hardcoded secret rule for Java lang. Here we follow the existing PHP rule and catch on non-trivial strings (and char arrays) assigned to suspect variable names like PASSWORD or apiKey.

We do not check for usage as a leaked password in source code is bad practice, whether or not it is in use.

Relates to #197

Checklist

• I've added a snapshot that shows my rule works as expected.
• My rule has adequate metadata to explain its use.
• PR title follows Conventional Commits format

[gitguardian]

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secret in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
8500090	Triggered	Generic High Entropy Secret	4c5b963	tests/java/lang/hardcoded_secret/testdata/main.java	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secret safely. Learn here the best practices.
3. Revoke and rotate this secret.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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<sup>🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.

Our GitHub checks need improvements? Share your feedbacks!</sup>

[elsapet]

GitGuardian warning is expected as we use realistic test data here