allow specifying file extension in `mktemp`

[simeonschaub]

It would be quite useful in many cases to allow the user to specify a specific file extension in mktemp. Looking at the implementation of mktemp, I don't think this should be too difficult to add.

[maleadt]

Ref #13969

[kbarros]

I guess as a workaround one could use mktempdir(). Inside that directory, there is freedom to create files with arbitrary names?