Add an `EndpointID` class to enforce the naming rules that we support
for actuator endpoints. We now ensure that all endpoint names contain
only letters and numbers and must begin with a lower-case letter.

Existing public classes and interfaces have been changes so that String
based `endpointId` methods are deprecated and strongly typed versions
are preferred instead. A few public classes that we're not expecting
to be used directly have been changed without deprecated methods being
introduced.

See gh-14773