Currently we have only two strategies of validating schema:

• strict - all fields must be exactly the same
• selective - ignore extra fields addded to given that are not present in expected, otherwise the same as strict

The goal is to add third option that will be aligned with a seamless schema evolution.
Here are the rules:

• if schemas are the same, return true
• if given schema has less fields than expected schema, return false
• if given schema is making a nullable field non-nullable, return false
• if given schema is making a non-nullable field nullable, return true
• if given schema is changing the type of a field, return false
• if given schema is adding a field, return true

There should be also dsl function:

schema_validate(Schema $expected, Schema $given, SchemaValidator = new StrictSchemaValidator()) : bool;

Additionally (it's a small bc break) we should change the order of argumetns in SchemaValidator::isValid()

Now:

• `isValid(Schema $given, Schema $expected) : bool;

Should be

• isValid(Schema $expected, Schema $given) : bool