Improve onboarding by having most popular backends integrated by default

[jamescmartinez]

from #19

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I could see that being an alternate configuration for 2-3 built-in backends. The existing method of configuration would still work for power users, but passing a string would use a backend with default configuration:

Zero config

Possibly defaults to sqlite backend so no external service is needed to start using OpenWorkflow:

const ow = new OpenWorkflow();

Backend string option

Looks for OPENWORKFLOW_BACKEND_DB environment var:

const ow = new OpenWorkflow({ backend: 'postgres' }); // or { backend: 'sqlite' }

Backend option (current)

Customize backend with other options (like namespaceId) or use community backend:

const backend = await BackendPostgres.connect(postgresUrl);
const ow = new OpenWorkflow({ backend });

As far as tree-shaking (relevant to serverless environments), the backend string is a runtime config and wouldn't be able to tree-shake. But the backends are very minimal so not a big concern.

Originally posted by @nathancahill in #19 (comment)