Unify ColumnType and CqlType

[piodul]

Currently, the driver has two complex but similar types: ColumnType and CqlType. They are both supposed to represent a CQL type, but have slight differences:

• ColumnType is used in metadata of prepared statements and responses to queries. It does not include information about whether collections and UDTs are frozen. The type definitions are self contained - if it represents a complex type that contains a UDT somewhere inside it, it has the whole definition of the UDT inline.
• CqlType is used in ClusterMetadata - it is computed based on the information in the system tables. It does include information about whether collections and UDTs are frozen. Until recently (changed in https://github.com/scylladb/scylla-rust-driver/pull/649) information about UDTs was not inlined, but now it is; however, if metadata fetch returns inconsistent metadata and it does not have a definition of a UDT that was needed inside some CqlType, then a placeholder MissingUserDefinedType will be put instead.

Those types have very similar names and function, so it might make sense to unify them. This will help write user logic that is supposed to operate on both representations. We will have to account for the differences:

• Presence vs. absence of information about being frozen: the newly introduced type could be generic over the "frozen flag" type. The "frozen flag" type could be () vs. bool, or two new enums isomorphic to those types that express the intent better.
• Handling the MissingUserDefinedType case:
  • The simplest way out would be to put Result<_, MissingUserDefinedType> in the unified type. However, this would force the users that need to operate on the existing ColumnType representation to handle the Result, even though it would never be an error, so that would introduce unnecessary complication and would be ugly.
  • An improvement upon the previous would be to convert MissingUserDefinedType into a generic parameter, like in the case of the "frozen flag" - in case of ColumnType it would be an uninhabited type. Slightly better, but the Result is still there.
  • (Preferred) Get rid of MissingUserDefinedType altogether! I insisted on handling the MissingUserDefinedType error like this to make the driver more robust; if we didn't have it then we would have to fail the whole metadata fetch and probably fail the creation of Session. However, if we move the cluster metadata module out of the driver like described in https://github.com/scylladb/scylla-rust-driver/issues/595, then it should be fine to fail the whole metadata fetch as it won't reduce availability. Therefore #595 is a requirement for this solution.

[roydahan]

If we decided that it shouldn't be part of 0.12.0, please comment and describe the decision and dependency and change the Milestone.

[piodul]

We discussed it offline with @muzarski and decided to go with the last option on the "Handling the MissingUserDefinedType case". It introduces a dependency on https://github.com/scylladb/scylla-rust-driver/issues/595, which is currently being worked on and targets 0.13.0, so I'll move this one to 0.13.0, too.