Adding support for prepared statements in Swift driver

[shruti-p-s]

Adding support for prepared statements in Swift driver.

Motivation:

The client had no way to prepare CQL queries server-side and reuse them. Every query was parsed fresh on each execution. For encrypted workloads, this also meant write-path validation could only check one direction — encrypted values bound to wrong columns — because inline queries lack column name metadata for positional parameters.

Modifications:

• PreparedStatement wrapper class around the C driver's CassPrepared*, exposing parameter count and column name metadata. - prepare() and execute(prepared:) on CassandraSession protocol, with both EventLoopFuture and async variants, plus Codable decoding overload.
• Bidirectional encryption validation using prepared statement parameter metadata — catches both plaintext→encrypted and encrypted→plaintext mismatches.
• 5 integration tests (4 ELF, 1 async) and 3 encryption integration tests using prepared statements.

Result:

Users can prepare queries once and execute them repeatedly with different parameters, avoiding repeated server-side parsing. Encrypted workloads get full bidirectional write-path validation when using prepared statements with column registration.