Default to allowDynamicBackends

[guybedford]

The JS SDK allowDynamicBackends(true) is entirely independent from the customer-level configuration. This has introduced some confusion to users in the past where the distinction between these configurations is not clear.

The proposal here is to effectively default the JS SDK to allowDynamicBackends(true), and then if attempting to dynamically create a backend and there is no host support to directly provide the error that dynamic backends are not enabled for the service and some help on how to enable it.

Then we can effectively deprecate this call on Fastly experimental.

Thinking of possible counter arguments, the main one I can think of is if a user wanted to ensure they were guarding against mistakenly using dynamic backends. But unless I'm mistaken, that seems at first glance to me not to be a major use case.