PostgreSQL: Date/Time functions and types

I'm thinking of adding a module to the `PostgreSQL` part of Esqueleto that will handle as many of the functions and types from the [Date/Time Functions and Operators](https://www.postgresql.org/docs/12/functions-datetime.html) section as I feel is doable for a first iteration.
Probably name it something like `Database.Esqueleto.PostgreSQL.DateTime`

Thought I'd make an issue to discuss this addition. I'd probably leave the operators be, for now, because of the complexity of the possible type pairs.

Start by reviewing the PostgreSQL Date/Time Functions and Operators documentation and the existing PostgreSQL portion of Esqueleto. Consider the proposed Database.Esqueleto.PostgreSQL.DateTime module and define a concrete first-iteration set of functions and types; done means that selected support is implemented while operators remain out of scope.

Written by the indexing model from the issue text.