ZEPPELIN-3037 Configure Http Request Header Size Limit for Jetty

[mayor-subatom]

What is this PR for?

In some deployment scenarios it is necessary to increase jetty.request.header.size, which default value is 8192. This will reduce the chance of HTTP Error 413 Request entity too large.
There should be a mechanism to configure this setting.

What type of PR is it?

[Feature]

What is the Jira issue?

• https://issues.apache.org/jira/browse/ZEPPELIN-3037

How should this be tested?

• There is an integration test (included) for testing this feature.
• To test manually, after increasing setting, make any http request to zeppelin with a request header bigger than 8K , you should not get an HTTP Error 413 Request entity too large.

Questions:

• Does the licenses files need update? No.
• Is there breaking changes for older versions? No.
• Does this needs documentation? Yes.