Implement TIMESTAMP/TIMEOUT for inserts

[Jasperav]

The ORM does not support TIMESTAMP and TIMEOUTs for inserts yet: https://docs.scylladb.com/getting-started/dml/#insert-statement.

This can be implemented by following these steps:

1. Add the new method name in query_ident.rs. The query ident mod can be used by other users to interact with always up to date methods, so they never have to type the method names themselves and run into run time errors
2. Add the new insert method somewhere in write_struct.rs, after the insert method (see line 177).
3. Add a test in lib.rs. I want each new change to be tested.

To see the changes of the generated code easily:

1. Change a line somewhere in build.rs. (there is no cargo:rerun-if-changed). This should trigger re-generation of the structs. The structs are in version control, so it should be easy to see what has been changed.
2. Run the test in lib.rs.