(Postgres) Add Annotation for Generated Expressions

[ivanvanderbyl]

This PR resolves #3168 to add support for generated columns as needed when using tsvector or similar extensions in Postgres.

Usage:

field.String("text_searchable_content").
	SchemaType(map[string]string{dialect.Postgres: "tsvector"}).
	Annotations(entsql.GeneratedAs("to_tsvector(...)")).
	Optional()

Todo:

• Updated schema migration template to add generated expressions

Follow up work:

Obviously for something like tsvector where you never read or write the column, this is suboptimal and a follow up PR should add a new field type for this use case, such as field.Generated("name").As("to_expression()") with the following properties:

• Does not add Setters or Getters for the field
• Field is immutable
• Field is optional for creates
• Generates a model.FieldName var

[ivanvanderbyl]

👋🏻 @a8m hey Ariel, I'd love to get your input on this, and any changes I can make to get it merged