Create filename parameter and get rid of attribute in FixedResultPipe

I've suggested this on other PR's before, but what about using an actual `Param` and get rid of this attribute? 😄

_Originally posted by @Laurens-makel in https://github.com/frankframework/frankframework/issues/11084#issuecomment-4782544936_

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Add a special parameter which holds the filename instead of using the `filename` or `filenameSessionkey` attributes.

May cause problems when you wish to replace a variable called `filename`.

Start by locating FixedResultPipe and reading how its filename and filenameSessionkey attributes are handled, along with the existing Param conventions. Replace the attribute-based filename handling with the proposed special parameter, account for variables named filename, and verify the affected behavior with the relevant tests.

Written by the indexing model from the issue text.