feat: initial client implementation

[andreabadesso]

Motivation

While integrating the mobile-wallet with SES, I've detected that the unleash-proxy-client-js library attempts to mutate the javascript prototype, which is forbidden by default by the sandbox

We could whitelist it, but since the lib has many features that we don't use, I thought it would be a better idea to create our own, with a limited subset of features that we actually use in our services and with no dependencies

Acceptance Criteria

• We should have a minimal version of an unleash client with only the methods we use
• We should be able to fetch the feature toggles
• We should be able to check wether a feature toggle is enabled or disabled for a given user
• We should use E-TAG to reduce bandwidth (by only downloading toggles if they changed)

Security Checklist

• Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.