Get ephemeral tokens for Partners

[lucmir]

Add the getEphemeralToken method to the Civic client so Civic (requester) customers will be able to secure the Partner interactions.

The ephemeral token is retrieved from a central service that is authenticated using a JWT based on the app specific ID and keys.

Usage example:

const civicSip = require('civic-sip-api');

const civicClient = civicSip.newClient({
    appId: 'ABC123',
    appSecret: APP_SECRET,
    prvKey: PRV_KEY,
});

civicClient.getEphemeralToken()
    .then((ephemeralToken) => {
      // use ephemeralToken in requests from Partners
    }).catch((error) => {
        console.log(error);
    });

[dmelosantos]

I see no changes to be made.

[slaphead]

Would it be possible to add a use case description here and show how it would be used on the front end? I think it would help anyone coming here to understand when to use this token?