Content modifier processor should allow to set timestamp

[OlafTitz]

Is your feature request related to a problem? Please describe.
I tried to replace a Regex parser with a Content modifier processor (see #11688) but that is not able to set the timestamp.

Describe the solution you'd like
Content modifier should support setting the timestamp with the time_key and related parameters like the Regex parser.

Describe alternatives you've considered
Finally I implemented that in Lua but that requires Lua extensions.

Additional context
That came up while experimenting with the solutions described in #11687.

[cosmo0920]

Adding timestamp support to the content modifier processor is not appropriate because:

• It violates the separation between record transformation and metadata handling
• It introduces ambiguity in timestamp ownership
• It negatively impacts performance and simplicity
• The functionality is already properly handled by parsers and filters

In short:

Timestamp handling belongs to parsing or filtering stages, not to processors.