Include extensions in to the Discovery Service classpath and include support of extensions with native libraries

[arxioly]

Is your feature request related to a problem? Please describe.
A new "standard" way to enable extensions in APIML supports only including extension in to the API Gateway classpath. But we have APIML extension (Audit Log), which extends also Discovery Service, so it also needs to be added to the Discovery Service classpath.

Also we are working on a new extension which requires including native libraries to the java.library.path, and we would like to have a standard way to include our libraries to the LIBPATH. I'm not sure that we can use LIBRARY_PATH variable for that purpose.

Describe the solution you'd like

1. Create a similar to API Gateway environment variable, like ZWE_DISCOVERY_SHARED_LIBS which will include extensions to the Discovery Service classpath in start.sh. For example:

...
if [[ ! -z ${ZWE_DISCOVERY_SHARED_LIBS} ]]
then
    DISCOVERY_LOADER_PATH=${ZWE_DISCOVERY_SHARED_LIBS},${DISCOVERY_LOADER_PATH}
fi
...
-Dloader.path=${DISCOVERY_LOADER_PATH} \

2. Create new environment variable, like ZWE_GATEWAY_LIBRARY_PATH to include Extension's native libraries to the LIBPATH of the API Gateway. For example:

...
LIBPATH="$LIBPATH":"${LIBRARY_PATH}"
if [[ ! -z ${ZWE_GATEWAY_LIBRARY_PATH} ]]
then
    LIBPATH="$LIBPATH":"${ZWE_GATEWAY_LIBRARY_PATH}"
fi
...

Most important that both environment variables could be set in instance.env.

Describe alternatives you've considered
LPL team can create PR with these changes.

[CarsonCook]

PR to add the requested changes: #1987
PR to docs site: zowe/docs-site#1959
@arxioly please let me know if you have any comments, thanks!