[mlir][Transform] Add `updateConversionTarget` to `ConversionPatternDescriptorOpInterface`

This change adds a method to modify the ConversionTarget used during
`transform.apply_conversion_patterns` to the
`ConversionPatternDescriptorOpInterface`. This is needed when the TypeConverter
is used to dictate the dynamic legality of operations, as in "structural"
conversion patterns present in, for example, the SCF and func dialects.

As a first use case/test, this change also adds a
`transform.apply_patterns.scf.structural_conversions` operation to the SCF
dialect.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D158672