Add an source index to RESP3ParsingError

[Joannis]

We should add an index to the RESP3ParsingError, so that we can track where in the buffer parsing failed. This can be immensely helpful when trying to debug RESP3 values with a more complex structure.

Original Issue

          Could we add an index here? That way we can correlate the error back to a specific location in the received ByteBuffer. Definitely helps debugging should this ever trigger

Originally posted by @Joannis in https://github.com/swift-server/RediStack/pull/71#discussion_r1253053557