Support for runtime configuration

[radupurdea]

Currently, the configuration sent to Intercom is done on the plugin load method. This is an issue if you do not have the keys at build time, or hardcoding them anywhere is not desired.

It would be good if the configuration could be sent at runtime, for example exposing a loadConfiguration plugin method that accepts the Intercom configuration, and would just set the Api key.

The existing load method would ignore the apiKey if not present in the capacitor.config.json.