[admin] Added billing settings section.
[admin_options] Added settings fields for MxitVendorID and json service url for external references.
[functions_admin] Added mxpress_billing_section_text() method to render billing section.
[single-kzm_book] Used mxpress optins for serviceURL and item Type OID.
[admin_options] Added virtualItemTypeOID to mxpress options.
Merge pull request #3 from MongeziNombewu/features/improvements-for-everegg

Added billing mxpress optoins