[Enhancement] Introduce new config for allowlisting datasources URI.

[vamsimanohar]

Is your feature request related to a problem?
In order to restrict datasource URIs and prevent any kind of ssrf attacks, a new config for allowlisting datasources URI should be introducted.

What solution would you like?
plugins.query.datasources.uri.allowhosts can be the new opensearch.yml config to restrict only domains which follow the provided regex pattern.