Add client option to set websocket as default persisted transport preference

[owenpearson]

add a new option (something like ClientOptions.websocketsFirst) which makes the client try websockets first if there isn't already a persisted transport preference and persist comet as a transport preference if websocketsFirst is true and the library fails to connect with websocket.

┆Issue is synchronized with this Jira Task by Unito

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➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3629

[VeskeR]

in ably-js v2 in https://github.com/ably/ably-js/pull/1645 PR we decided to go with another approach: we removed XHRStreaming transport, and replaced upgrade mechanism with a websocket + base fallback transport mechanism whereby no two transports may be simultaneously connected.

@owenpearson Would you be happy to close this issue?