Harmonize registration of reflection hints

[snicoll]

We got some feedback in #28977 that led us to reconsider some of the constructs in ReflectionHints and friends. Looking at the usage we've decided to:

• Limit the ExecutableMode to a single value, with the ability to determine if a user-provided value is relevant or if it's already included in the configured one
• Provide consistent registration methods that allows to set the ExecutableMode without having to customize the builder
• Make sure that methods that do not take a builder and those who take one without mutating it generate consistent hints