[mlirTranslateMain] Add a customization callback.

mlir-translate and related tools currently have a fixed set
of flags that are built into Translation.cpp.  This works for
simple cases, but some clients want to change the default
globally (e.g. default to allowing unregistered dialects
without a command line flag), or support dialect-independent
translations without having those translations register every
conceivable dialect they could be used with (breaking
modularity).

This approach could also be applied to mlirOptMain to reduce
the significant number of flags it has accumulated.

Differential Revision: https://reviews.llvm.org/D120970