Support running in offline mode (e.g. bootstrap feature toggles from a file)

[vgrigoruk]

Describe the feature request

Unleash NodeJS sdk has the following functionality that supports running in "offline" mode:

• https://docs.getunleash.io/reference/sdks/node#bootstrap
• https://docs.getunleash.io/reference/sdks/node#custom-repository
• setting refreshInterval = 0 disables refreshing the toggles from the upstream URL.

Using these options, it is possible to run the the client w/o network access to Unleash API. It would be nice if unleash-client-go also supports this.

Background

Running the SDK in offline mode is necessary to support the following use-cases:

• running the apps in offline environments, where toggle updates only occur when a new version of the code is deployed
• support determinism in local development and CI environment while keeping the setup simple (avoid adding unleash-edge running in "offline" mode to local dev/test environment).

Solution suggestions

No response

[FredrikOseberg]

@vgrigoruk The Golang SDK also supports bootstrapping from a file. See this section of the README.md. It should also support setting refreshInterval to 0 via configOptions.