[ML] Make the URL of the ML C++ Ivy repo configurable (#48702)
[ML] Make the URL of the ML C++ Ivy repo configurable (#48702)

At present the ML C++ artifact is always downloaded from
S3.  This change adds an option to configure the location.

(The intention is to use a file:/// URL to pick up the
artifact built in a Docker container in ml-cpp PR builds
so that C++ changes that will break Java integration tests
can be detected before the ml-cpp PRs are merged.)

Relates elastic/ml-cpp#766
robin13/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,robin13/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,HonzaKral/elasticsearch,uschindler/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,uschindler/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,HonzaKral/elasticsearch,nknize/elasticsearch,uschindler/elasticsearch,HonzaKral/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch