Schema metadata: support UDFs, UDAs & indexes

[wprzytula]

Currently, driver does not fetch nor store User Defined Functions, User Defined Aggregates nor indexes definitions (from system_schema.[functions, aggregates, indexes], correspondingly).

cpp-driver exposes API to examine those, so Rust driver should support this, too, for cpp-rust-driver.

[mykaul]

UDFs are not GA, should be a separate issue.

[wprzytula]

UDFs are not GA, should be a separate issue.

What about UDAs?

[wprzytula]

Are UDFs stable in Cassandra? If so, and the driver strives for Cassandra compatibility, then we should add them along those schema entities that are GA in ScyllaDB.

[mykaul]

Are UDFs stable in Cassandra? If so, and the driver strives for Cassandra compatibility, then we should add them along those schema entities that are GA in ScyllaDB.

I don't know, but our priority is according to what ScyllaDB supports, not what Cassandra.