feat: add email subscriptions

[wa0x6e]

This PR adds support for some of email subscriptions management through sequencer.

What will be supported (but same features still supported directly from envelop):

• create a new subscription
• update the subscription
• delete a subscription

What will not be supported (and will remain on envelop):

• Email verification

This PR will make use of new changes from snapshot-labs/snapshot.js#1081

This PR introduces 2 new writers:

• email-subscription: will create a new subscription when passing an email, else will update existing subscriptions
• delete-email-subscription: will delete a verified subscription

How to test

• update your local snapshot.js repo to use this branch feat: add email subscription snapshot.js#1081
• use your local snapshot.js repo with yarn link
• Update your .env with the correct mysql link to envelop db with ENVELOP_DATABASE_URL
• see the integration tests

Features

Create a subscription

await client.emailSubscription(wallet, wallet.address, {
    email: '[email protected]',
    subscriptions: []
  });  

This call from the SDK will create a new email subscription, if user is not subscribed yet.
In case the couple wallet.address/email already exist, it will return a email already subscribed error.

This call will create a new unverified email subscription. The user will then receive an email on the given email address asking the user to verify his email (verification will be done through envelop directly)

For test purpose, you have to connect to the database, and update the verified column to something greater than 0 to continue testing update and deletion.

The subscriptions value is empty, and always ignored on creation, and will default to everything.

Update a subscription

await client.emailSubscription(wallet, wallet.address, {
    subscriptions: ['summary']
  });  

This call from the SDK will update an existing subscription.

The email property is skipped (and uncessary), as there can be only one verified email associated to the wallet.

This call will fail with email not verified if email is not verified, or email not subscribed when not existing.

Delete a subscription

await client.deleteEmailSubscription(wallet, wallet.address, {});  

This call from the SDK will delete the active email subscription associated to the address.

Once again, email is not passed for privacy reason (and also unecessary), and this will delete the verified email subscription if any.

Call will fail with email not subscribed if there's not verified email subscription to delete.

For the email-subscription type, ipfs pinning will be skipped on email creation, and an empty string will be returned instead, as to not expose the user's email. (ipfs still pinning for email subscription update)

Todo

• release sdk from feat: add email subscription snapshot.js#1081
• Update PR to use new sdk version
• Update .env to add ENVELOP_DATABASE_URL (on mainnet only)

[wa0x6e]

Tests are failing for issue unrelated to this PR, should he fixed via #469

[ChaituVR]

The primary key can be just address no? because a user cannot use multiple emails. this will help us to use queries like INSERT ... ON DUPLICATE KEY UPDATE to insert or update email subscription in src/writer/email-subscription.ts

[wa0x6e]

An address can be associated to multiple email addresses, but only one can be verified.

I think this was done so an unverified address can subscribe again with another email address