db.pg: add support for prepared statements

[louis77]

PostgreSQL has two separate calls to support prepared statements. This PR adds the following two public functions to db.pg:

pub fn (db DB) prepare(name string, query string, num_params int) !
pub fn (db DB) exec_prepared(name string, params []string) ![]Row

plus one to handle an empty result to check for an error when prepare is called.

Also an appropriate test has been added.

[huly-for-github]

Connected to Huly®: V_0.6-21873

[spytheman]

Excellent work.

[felipensp]

Nice work!