[mlir:Bytecode] Add initial support for dialect defined attribute/type encodings

Dialects can opt-in to providing custom encodings by implementing the
`BytecodeDialectInterface`. This interface provides hooks, namely
`readAttribute`/`readType` and `writeAttribute`/`writeType`, that will be used
by the bytecode reader and writer. These hooks are provided a reader and writer
implementation that can be used to encode various constructs in the underlying
bytecode format. A unique feature of this interface is that dialects may choose
to only encode a subset of their attributes and types in a custom bytecode
format, which can simplify adding new or experimental components that aren't
fully baked.

Differential Revision: https://reviews.llvm.org/D132498