textlogger: optionally turn off header

[pohly]

What this PR does / why we need it:

Turning off the header is useful for converting structured log parameters (e.g. error + message + key/value pairs from the Kubernetes ErrorHandler API) into a single string.

See

Which issue(s) this PR fixes:
Fixes #429

Special notes for your reviewer:

Needed for https://github.com/kubernetes/kubernetes/pull/135419/files#r2576981852

Release note:

textlogger: WithHeader option enables suppression of the header.

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[pohly]

/assign @harshanarayana