Add library instrumentation for servlet

[laurit]

Supersedes #15188

[laurit]

I renamed testing to javaagent-testing for some reason this file didn't register as being moved :(

[laurit]

Currently ServletRequestContext is in internal package we have to either move it out of the internal or alternatively we could change the signature to UnaryOperator<SpanStatusExtractor<HttpServletRequest, HttpServletResponse>>. The wrappers are there mostly so we could share code between servlet 3 & 5 instrumentations. ServletResponseContext has some fields but ServletRequestContext doesn't have any and is probably there only for symmetry. If we don't use the wrappers in the signature then the downside is that conversion will be needed

    builder.setStatusExtractor(
        inputExtractor -> {
          SpanStatusExtractor<HttpServletRequest, HttpServletResponse> outputExtractor =
              statusExtractor.apply(
                  (spanStatusBuilder, request, response, error) ->
                      inputExtractor.extract(
                          spanStatusBuilder,
                          new ServletRequestContext<>(request),
                          response == null ? null : new ServletResponseContext<>(response),
                          error));
          return (spanStatusBuilder, requestContext, responseContext, error) ->
              outputExtractor.extract(
                  spanStatusBuilder,
                  requestContext.request(),
                  responseContext == null ? null : responseContext.response(),
                  error);
        });

@trask which way would you prefer?

[laurit]

not using otelJava because that would apply to the main source too which would break the instrumentation for libraries that only work with java 8 when tests are run with -PtestLatestDeps=true. The same approach could be used to get rid of the javaagent-testing module and move the tests back to javaagent.