Create LaunchDarkly feature flag

[matallui]

What

Setup a feature flag for the "Vault - Remote Mode v1" feature.

How

• Create feature in LaunchDarkly
• Add helper functions to read flag

References

https://github.com/MetaMask/contributor-docs/blob/main/docs/remote-feature-flags.md

[matallui]

manifest overrides, for local dev:

{
  "_flags": {
    "remoteFeatureFlags": {
      "vaultRemoteMode": true
    }
  }
}