Common settings for different validators of the same rule

[AndreuCodina]

Sometimes we repeat a customization or condition for each validator of a Rule.

int? age = 10;
Validate.That(age)
    .WithMessage("The age must be greater than or equal to 18")
    .NotNull()
    .WithMessage("The age must be greater than or equal to 18")
    .GreaterThanOrEqual(18);

We could customize them in the rule creation:

int? age = 10;
Validate.That(age, message: "The age must be greater than or equal to 18")
    .NotNull()
    .GreaterThanOrEqual(18);

We can add other settings, as for example:

Validate.Field(age, message: "Error message", when: true, fieldDisplayName: "Customer age", error: new UnderageError())

We have to validate possible conflicts, such as specifying a message and also an error with a message.
These settings could be saved in ValidationContext.