filebeat: fix Redis auth method to support username/password

[ma2sql]

Proposed commit message

filebeat: fix Redis auth method to support username/password

• WHAT: Updated the Redis authentication logic within the Filebeat input module to correctly handle username/password combinations. The AUTH command has been improved to use both username and password.
• WHY: The previous implementation attempted to authenticate using only the password, causing failures. Redis authentication requires two arguments: a username and a password. This update resolves the issue.
  Reference: Redis AUTH Documentation

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Author's Checklist

• Verify correct authentication with username/password.
• Validate no impact on existing configurations.

How to test this PR locally

Run redis_integration_test.go to verify changes.

Related issues

• [filebeat] Add SSL and AUTH username support for Redis input #40111

Use cases

Screenshots

Logs

[cla-checker-service]

💚 CLA has been signed

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @ma2sql? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[pierrehilbert]

@lalit-satapathy it could be interesting to have someone from your team also reviewing this PR.

[rdner]

This change needs a changelog entry in https://github.com/elastic/beats/blob/main/CHANGELOG.next.asciidoc

[ma2sql]

@rdner CHANGELOG.next.asciidoc added, thanks!

[rdner]

Looks good to me in the current state.

As was pointed out, we use the same approach in the Metricbeat module here

beats/metricbeat/module/redis/redis.go

Line 218 in 6e3634f

rd.DialPassword(password),

[rdner]

/test