TS-2741: add a new server intercept example plugin

Add a new example plugin to demonstrate the use of TSHttpTxnServerIntercept
to proxy origin requests to a secondary server.

Document TSHttpTxnServerIntercept, and update surrounding documentation
that references it.

Add a new test, test-server-intercept, that combines the example
plugin and jtest to do an end-to-end test of the intercept API.