Move the httpclient test server utility classes to a jtreg library

The tests under test/jdk/java/net/httpclient use some test server specific classes/APIs. These currently reside in unnamed packages, mostly under test/jdk/java/net/httpclient/http2/server directory.
Given that these are in unnamed packages, it has been noticed that it isn't easy to refer to them from other test utility classes. Moving them to a jtreg library and using a proper package name would help solve this issue. This will also allow any newer test specific utility classes to start refering to them at compile time.