Rename SessionIdGenerationStrategy to SessionIdGenerator

[marcusdacoregio]

We should change the name of the interface SessionIdGenerationStrategy to SessionIdGenerator for a couple of reasons:

• There is no need for the Strategy suffix
• The name is not too clear, SessionIdGenerator gives a better idea of what it is for
• Tomcat has a SessionIdGenerator and WebFlux uses IdGenerator, so we would not deviate too much

[rishiraj88]

Nice points by @marcusdacoregio .