Use endpoint as default connection option (ADR-119)

[maratal]

Need implementation for ADR-119[1], which specifies the client connection options to update requests to the endpoints implemented as part of ADR-042[2].

The endpoint may be one of the following:

a routing policy name (such as main)
a nonprod routing policy name (such as nonprod:sandbox)
a FQDN such as foo.example.com

The endpoint option is not valid with any of environment, restHost or realtimeHost, but we still intend to support the legacy options.

If the client has been configured to use any of these legacy options, then they should continue to work in the same way, using the same primary and fallback hostnames.

If the client has not been explicitly configured, then the hostnames will change to the new ably.net domain when the package is upgraded.

(copied from ably/ably-js#1973)

ADR: https://ably.atlassian.net/wiki/spaces/ENG/pages/3428810778/ADR-119+ClientOptions+for+new+DNS+structure

Spec PR: ably/specification#213

ably-js implementation: ably/ably-js#1973

ably-go implementation: ably/ably-go#679

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