[Heartbeat] Add stringer redaction to synthexec cmd

[emilioalvap]

Proposed commit message

Redact synthetics cmd output to prevent logging of user-defined parameters.

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

N/A

How to test this PR locally

1. Build heartbeat locally.
2. Add a browser monitor with any playwrightOpts or params.
3. Run and check logs show playwrightOpts or params as REDACTED

[elasticmachine]

Pinging @elastic/obs-ds-hosted-services (Team:obs-ds-hosted-services)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @emilioalvap? 🙏.
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-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[vigneshshanmugam]

LGTM