Allow for full url specification

There are cases in which app search is being an API gateway or reverse proxy that benefits from allowing the consumer of the app-search client to specify what the end point should be.

This recently came up on a search-ui implementation where just the production environment was going to be behind a API gateway. This would require implementing a custom handler/middleware in search-ui which would be better to simply have the string concatenation for the endpoint overwritten or specified by the consumer of the SDK.

https://github.com/elastic/app-search-javascript/blob/master/src/client.js#L50