Introduce "server.servlet" prefix for servlet specific properties

[bclozel]

This change would break the current arrangement with ServerProperties and nest all Servlet specific keys under a servlet prefix.

Current key	New key
server.address
server.compression
server.connection-timeout
server.context-parameters.*	server.servet.context-parameters.*
server.context-path	server.servlet.context-path
server.display-name
server.max-http-header-size
server.max-http-post-size
server.error.[...]	should we move this to an "mvc" namespace?
server.jetty.[...]
server.jsp-servlet.[...]	server.servlet.jsp.[...]
server.port
server.session.tracking-mode	warning: type javax.servlet.SessionTrackingMode
server.server-header
server.servlet-path	server.servlet.path
server.use-forward-headers
server.session.[...]
server.ssl.[...]
server.tomcat.[...]
server.undertow.[...]

Servlet specific properties should not apply when an application is deployed on a reactive runtime (even if it's a servlet container); in that case, we could silently ignore those or fail hard at startup.

New runtimes will be introduced, such as RxNetty and Reactor Netty (both based on netty). Configuration keys should be added as well.

Relates to #3659 and #4818

[snicoll]

jsp-servlet always bugged me, can we go with spring.servlet.jsp please?

[bclozel]

In that case, it would look like:

• server.servlet.jsp.class-name
• server.servlet.jsp.init-parameters.*
• server.servlet.jsp.registered

[snicoll]

See also #3016