Fully describe the Opaque type, and remove ByteBuffer

The byteBuffer type is superfluous since it can be represented by an array of uint8. It wasn't really implemented in several places anyway, but we can officially remove any remaining references.

Bytebuffer exists in a few places (Franca and Thrift) and in the Franca user guide it is as something that is an unknown sequence of bytes (aka BLOB), which we can now use Opaque for.

Byte-buffer could reasonably be interpreted as something that also might have only the structure of a sequence of bytes, and it's also not really clear if "buffer" implies some particular semantic meaning beyond array.

The 'Opaque' type has been conceived in IFEX instead to better indicate that something either has an advanced structure but that it cannot (or is chosen not to) have a detailed represented in the IFEX core IDL description (possibly augmented by some layer that is able to have an even more advanced description than the features of the IFEX core IDL). It can also be used in contexts where the type is simply "unknown" (however, don't confuse that with IFEX Variant type, which is "any one of a _known_ list of types".

Opaque type is also likely to be transferred as just a sequence of bytes, so technically it can replace byteBuffer without problem, but it can indicate some slightly deeper meaning.