HPCC-12167 Add WsTopology.TpGetServicePlugins

This new method is used to report ESP Service Plugins.
The information about ESP Service Plugins will be added
into esp.xml by another fix (for each ESP service plugin,
an EspService/@servicePlugin and, if needed, an EspService/
@servicePluginType are added into esp.xml). This fix
reads the information and return it in the WsTopology.
TpGetServicePlugins.

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>
HPCC-12167 Clean code based on review

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>
Merge pull request #6444 from wangkx/h12167b

HPCC-12167 Add WsTopology.TpGetServicePlugins

Reviewed-By: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>