Adds Flagsmith adapter

[tiagoapolo]

This pull request introduces a new provider adapter for the Flags SDK that integrates with Flagsmith. The changes include adding configuration files, updating dependencies, and creating the necessary code for the adapter.

Flagsmith documentation: https://docs.flagsmith.com/clients/next-ssr

Usage

import { createFlagsmithAdapter } from '@flags-sdk/adapter-flagsmith';
import { flag } from 'flags';

// Create the Flagsmith adapter
const flagsmithAdapter = createFlagsmithAdapter({
  environmentID: 'your-environment-id',
  // Optional: Add any other Flagsmith configuration options
  // See: https://docs.flagsmith.com/clients/javascript/
});

// Define your flags
const myFeatureFlag = flag({
  key: 'my-feature',
  adapter: flagsmithAdapter,
});

// Use the flag in your application
const flag = await myFeatureFlag();

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[dferber90]

Thank you for your contributions so far! I left a bit of early guidance

[dferber90]

Do you work for Flagsmith? If we merge this PR we'd also put up some actual docs on flags-sdk.dev. Would you be wiling to link to our Flags SDK and those docs from https://docs.flagsmith.com?

[dferber90]

The pattern other adapters follow is this

const myFeatureFlag = flag({
  key: 'my-feature',
  adapter: flagsmithAdapter.getFeature(),
});

Configuring SDKs per feature flag
We are able to pass additional information for that specific flag, for example if the key of the flagsmith flag differed from the key of the Flags SDK flag you could do adapter: flagsmithAdapter.getFeature({ key: "foo" })

Lazy mode
We usually export a default adapter, which is initialized lazily, so people can use the default adapter without any configuration. For this to work, the Flagsmith adapter needs to respect a default set of environment variables, eg FLAGSMITH_ENVIRONMENT_ID.

import { flagsmithAdapter } from "@flags-sdk/flagsmith"

const myFeatureFlag = flag({
  key: 'my-feature',
  adapter: flagsmithAdapter.getFeature(),
})

You can check the implementation of the Statsig/LaunchDarkly adapters as a reference.