SOLR-15525: Read ZK credentials from a file specified using a system property instead of exposing plain-text credentials

[thelabdude]

https://issues.apache.org/jira/browse/SOLR-15525

Description

Added new Java system property zkDigestCredentialsFile to allow loading of zk creds from a file instead of from system properties.

Tests

Added to existing unit test VMParamsZkACLAndCredentialsProvidersTest to use the new zkDigestCredentialsFile feature.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide

[HoustonPutman]

Clean and Simple 👍

[madrob]

Yea but I didn’t test it 😅

[thelabdude]

@madrob ~ forbidden check didn't like your suggested change:

Forbidden method invocation: java.util.Properties#store(java.io.OutputStream,java.lang.String) [Properties files should be read/written with Reader/Writer, using UTF-8 charset. This allows reading older files with unicode escapes, too.]
  in org.apache.solr.cloud.VMParamsZkACLAndCredentialsProvidersTest (VMParamsZkACLAndCredentialsProvidersTest.java:302)
> Task :solr:core:forbiddenApisTest

It looks like it wants a FileWriter like I had originally.