Keymanager APIs: optin flag for keystore exports API

[james-prysm]

🦄 Feature Tracking

Current status (opt-in or opt-out):
opt-in

Opt-in feature flag name:
TBD

Feature Description

to align with other clients and be more secure we should have an opt in flag to allow keystore exports while running the keymanager APIs

currently as long as the web or rpc flags are used on the validator client and an authentication token is used the user can have access to keymanager APIs however there is no way to export keystores via API as of yet due to security concerns. This issue will be used to track process on this API.

ethereum/keymanager-APIs#75