Flight SQL filter pattern improvements

[devinrsmith]

The Flight SQL filter pattern spec leaves a lot to be desired. That is, there does not seem to be any potential for escaping. In particular, the _ (which is pretty common in DH table names) can't be matched literally, at least according to the current specification: https://github.com/apache/arrow/blob/apache-arrow-18.0.0/format/FlightSql.proto#L1179-L1186. This may be something we want to raise with the Arrow project.

We may also want to increase our testing of Unicode filter patterns.