[LOGBACK-1688] Fix request log on Jetty 10.0.x

[zUniQueX]

The compiled JAR for logback-access:1.3.0 doesn't fully work with Jetty 10.0.x.

When decompiling the JAR with javap, the following Methodref is used in the JettyModernServerAdapter#buildResponseHeaderMap() method:

#49 = Methodref #14.#50 // org/eclipse/jetty/server/Response.getHttpFields:()Lorg/eclipse/jetty/http/HttpFields;

#50 = NameAndType #51:#52 // getHttpFields:()Lorg/eclipse/jetty/http/HttpFields;
#51 = Utf8 getHttpFields
#52 = Utf8 ()Lorg/eclipse/jetty/http/HttpFields;

Since the return type was changed to HttpFields.Mutable in Jetty 10.0.x, the method isn't found and the following exception is thrown:

java.lang.NoSuchMethodError: 'org.eclipse.jetty.http.HttpFields org.eclipse.jetty.server.Response.getHttpFields()'

However, in both Jetty versions the return types implement Iterable<HttpField>, so we can resolve the correct method via reflection and cast the result to Iterable<HttpField>.

Additionally, I think it's a good idea to make the makeJettyServerAdapter method protected to give users the flexibility to disable the reflection usage, if this gets a performance issue.

CC @joakime Maybe you can take a look at this, if this is correct or if I'm missing something.

[zUniQueX]

Anything I can do to make it easier getting this merged?