Add search.remote.{name}.http_addresses setting

[clintongormley]

Users should be able to specify a preferred list of http_addresses when configuring a remote cluster for cross-cluster search.

Question: should this be _addresses or _urls? The nodes may be behind HTTPS instead of HTTP, so should they be specifying https://10.0.0.1:9200? or should we add search.remote.{name}.https: true|false?

[clintongormley]

@s1monw what do you think?

[s1monw]

why would we need this if we can get this through the API added in #23925

[s1monw]

I think we should keep it simple for now and just use what the node info gives us. @clintongormley can you close this if you are ok with it.