Feat: String validations

[alexgarzao]

With this task must be allowed to use the following validations in string fields:

• eq (equal): must be equal to the specified value
• eq_ignore_case (equal ignoring case): must be equal to the specified value (ignoring case)
• ne (not equal): must not be equal to the specified value
• ne_ignore_case (not equal ignoring case): must not be equal to the specified value (ignoring case)
• len (length): must have the following length
• max (max): must have no more than max characters
• min (min): must have no less than min characters
• in (in): must be on of the following values
• nin (not in): must not be on of the following values
• required (required): is required