Support raw query for order

[miry]

In rails I have a feature to order columns by RANDOM() with sample: User.order(RANDOM()).first.
Currently order is expecting the field references and produces the error:

> Order.order("RANDOM()")

Marten::DB::Errors::InvalidField: Unable to resolve 'RANDOM()' as a field. Valid choices are: user_id, created_at, updated_at, id.
    lib/marten/src/marten/db/query/sql/query.cr:967:13 in 'raise_invalid_field_error_with_valid_choices'
    lib/marten/src/marten/db/query/sql/query.cr:739 in 'raise_invalid_field_error_with_valid_choices:allow_many'
    lib/marten/src/marten/db/query/sql/query.cr:881:15 in 'get_field_context'
    lib/marten/src/marten/db/query/sql/query.cr:1085:19 in 'verify_field'
    lib/marten/src/marten/db/query/sql/query.cr:1059:19 in 'verify_field:allow_many'
    lib/marten/src/marten/db/query/sql/query.cr:973:13 in 'solve_field_and_column'
    lib/marten/src/marten/db/query/sql/query.cr:263:32 in 'order'
    lib/marten/src/marten/db/query/set.cr:1143:11 in 'order'
    lib/marten/src/marten/db/model/querying.cr:685:13 in 'order'

[ellmetha]

Hey! 👋

This is definitely missing from the framework. I think that two possible things could be added:

1. We could add the ability randomly order a query set by using a specific character when using #order. For example:
   User.order("?")
2. We could add the ability to order a query set by specifying a raw SQL expression. For example:
   `User.order("RAND()")

These two options are not mutually exclusive. (1) makes it possible to randomize the order of records without ending up incorporating DB-specific expressions, which allows projects to remain more DB-agnostic. (2) could be a good option for more specific needs that are not supported by the framework out-of-the-box.