[PoC] First implementation Worker Availability

[GPaoloni]

Description

This PR is a complementary piece to techmatters/flex-plugins#597.

• Sets to true keepAvailable when assigning offline contacts to Offline workers.

[WIP]

• Do the same for transfers once we have a clearer idea on how to handle this (ongoing conversation).

Verification steps

This is deployed to the dev environment, so just changing the serverlessBaseUrl and test from the frontend is the easiest. This can be done by pasting in the Chrome Console:
Twilio.Flex.Manager.getInstance().serviceConfiguration.attributes.serverless_base_url = "https://serverless-9971-dev.twil.io"

[GPaoloni]

Asking review from the rest of the team so you are aware of this extra piece, but nothing really complicated.

[stephenhand]

.