[browser] HTTP service

[epixa]

In the browser, we need a basic HTTP service in the browser that exposes an HTTP request client to plugins that is preconfigured to send things like kbn-version and wire into the error handler.

This is related to #12464, though it isn't technically blocked by that since it needs to work with legacy endpoints anyway.

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This service should expose a browser client for simplifying HTTP requests. The long-term goal of this service/client is to replace the legacy kfetch service. In addition, this service should abstract away the commonalities of making HTTP requests as well as custom functionality:

• Setting default headers
• Automatic response parsing
• Default error handling
• Abortable API
• Convenience methods for REST operations