Add support on ColumnInformation for extracting additional information

[ivanceras]

Add support on ColumnInformation for extracting additional information

• default_value
• is_unique
• limit if any (ie: varchar(36) in mysql)
• comment
• referred column(if foreign key) of the foreign table ie: product.owner references users(user_id)

Use case:

The database admin ui - curtain I've been working on uses these additional information to create a generic UI for the generated fields based on the table schema.

[sgrif]

With the possible exception of limit, none of those fields are relevant to Diesel. You're certainly able to use Diesel to query information_schema in a similar fashion that we do internally, but the internals of diesel_codegen and diesel_infer_schema are not part of our public API and shouldn't be used directly.