[Change Proposal] Allow to define expected_event_types

[jsoriano]

Similar to #616, allow to define expected_event_types as part of allowed_values.

expected_* values in ECS are not mandatory, but we want to keep the validations by default to offer guidance to package developers, to use consistent values for the fields that have these recommendations. Though as we did with expected_values in #616, we could offer the possibility to override the values, to adjust them to the use case of the package.

Some previous conversations about this:

• [AWS] Migrate AWS package to ecs@mappings integrations#10223 (comment)
• Tests fail due to ECS expected values elastic-package#1472