Clarify support of "Expect: 100-continue" in filters/servlets

I have recently raised a [thread on the Tomcat Users Mailing list](https://www.mail-archive.com/users@tomcat.apache.org/msg134171.html) why servlet filters are not subject to expect continue handling. In detail, when a client sends this header filters and servlets only kicked in *after* the container has processes the expect continue chain. Making it impossible to send redirects or other status codes prematurely w/o reading the body.
In contrast to that Tomcat internal valvles fully handle it automatically.

Please clarify whether a servlet container shall support the handling of expect continue in filters/servlets and how.

Start with the linked Tomcat Users Mailing list thread and the Jakarta Servlet specification text concerning Expect: 100-continue. Determine whether filters and servlets are required to handle it before the request body is read, and document the expected behavior and how premature responses should work.

Written by the indexing model from the issue text.