[mlir] Extend BufferAssignmentTypeConverter with result conversion callbacks

In this PR, the users of BufferPlacement can configure
BufferAssginmentTypeConverter. These new configurations would give the user more
freedom in the process of converting function signature, and return and call
operation conversions.

These are the new features:
    - Accepting callback functions for decomposing types (i.e. 1 to N type
    conversion such as unpacking tuple types).
    - Defining ResultConversionKind for specifying whether a function result
    with a certain type should be appended to the function arguments list or
    should be kept as function result. (Usage:
    converter.setResultConversionKind<MemRefType>(AppendToArgumentList))
    - Accepting callback functions for composing or decomposing values (i.e. N
    to 1 and 1 to N value conversion).

Differential Revision: https://reviews.llvm.org/D85133