Add option to use pointers instead of sql.Null* types

[mvrhov]

This adds support to use pointers instead of sql.Null* types. I'd like to get general comments before having a go at adding a few tests.

[alecbz]

There's already a way to configure the use of different types than the standard ones for fields, right? Could we expand that to support pointers vs. adding a dedicated config option just for pointers?

[heikkilamarko]

Hi! What is the status of this issue?
Is there any way to generate pointers instead of sql.Null* types yet?

[mvrhov]

Different version of this was implemented it's a bit annoying as you have to change the config and add the types you want to have pointers to...
This is my PostgreSQL list for now.

overrides:
  - db_type: "pg_catalog.timestamp"
    go_type:
      import: "time"
      type: "Time"
      pointer: true
    nullable: true
  - db_type: "pg_catalog.timestamptz"
    go_type:
      import: "time"
      type: "Time"
      pointer: true
    nullable: true
  - db_type: "text"
    go_type:
      type: "string"
      pointer: true
    nullable: true
  - db_type: "ltree"
    go_type:
      type: "string"
      pointer: true
    nullable: true
  - db_type: "jsonb"
    go_type:
      import: "encoding/json"
      type: "RawMessage"
      pointer: true
    nullable: true
  - db_type: "pg_catalog.int2"
    go_type:
      type: "int16"
      pointer: true
    nullable: true
  - db_type: "pg_catalog.int4"
    go_type:
      type: "int"
      pointer: true
    nullable: true
  - db_type: "pg_catalog.int8"
    go_type:
      type: "int64"
      pointer: true
    nullable: true
  - db_type: "pg_catalog.bool"
    go_type:
      type: "bool"
      pointer: true
    nullable: true
  - db_type: "pg_catalog.float4"
    go_type:
      type: "float32"
      pointer: true
    nullable: true
  - db_type: "pg_catalog.float8"
    go_type:
      type: "float64"
      pointer: true
    nullable: true