Improve type checks in SitesManager API

[sgiehl]

Description:

While working on another issue I came across the fact, that the SitesManager API currently does not check the type that is provided when creating or updating a measurable.

This makes it possible to create measurables with an invalid type, or update an existing measurable with one.

This PR therefor introduces a check if the provided type exists.
When updating an existing site, the type will only be checked if it differs from the previous one.
This is done, to ensure that a measurable that already has an invalid type (e.g. due to a removed plugin), is still update-able.

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed