Add support for parsing/printing dialect defined attributes. This also...

    Add support for parsing/printing dialect defined attributes. This also adds support for a pretty syntax for dialects attributes that is synonymous with the pretty syntax for dialect types. This cl also adds a new attribute 'OpaqueAttr' that allows for roundtripping attributes attached to unregistered dialects.

    Dialect attributes have the following syntax:
       dialect-attribute  ::= `#` dialect-namespace `<` `"` attr-data `"` `>`
       dialect-attribute  ::= `#` alias-name pretty-dialect-sym-body?

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PiperOrigin-RevId: 248344416