Add support for deletion of an installation (InstallationStore / MemoryInstallationStore)

[misscoded]

Summary

Fixes #1271

Adds deleteInstallation to the MemoryInstallationStore class, as well as to the InstallationStore interface.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[seratch]

Can you fix the same error for fetchInstallation method?

Suggested change

	if (query.isEnterpriseInstall && query.enterpriseId !== undefined) {
	if (installQuery.isEnterpriseInstall && installQuery.enterpriseId !== undefined) {

[misscoded]

Good catch on both of these. Will update!

[seratch]

Suggested change

	if (query.teamId !== undefined) {
	if (installQuery.teamId !== undefined) {

[seratch]

The changes are fine. While reviewing this, I noticed that memory store does not support user tokens. We may want to add some comments to DevDatabase like this:

// using a javascript object as a makeshift database for development
// storing user tokens is not supported
interface DevDatabase {
  [teamIdOrEnterpriseId: string]: Installation;
}

[stevengill]

Should we consider making this optional incase folks have created their own installationStores. By making it required, it would mean their installationStores are invalid when they upgrade.

[seratch]

Ah yeah, great point. Thanks.

[stevengill]

@seratch how does memoryStore not support userTokens? It stores the entire installation object which should contain the userToken. That would then get attached to context in bolt

[seratch]

@stevengill If the internal datastore supports user tokens too, the key should not be teamIdOrEnterpriseId. Your app can have multiple user tokens per workspace/org. The current interface stores only the latest installation for a workpace.

interface DevDatabase {
  [teamIdOrEnterpriseId: string]: Installation;
}

Your app may ask an admin to install the bot and ask other end users to install the app with a set of user token scopes (this is a common scenario). Your app may want to look up user tokens by a user ID as necessary. The simplest interface (which does not support historical data) would be like this:

interface DevDatabase {
  [teamIdOrEnterpriseId: string]: {
    bot?: Installation,
    [userId: string]: Installation
  }
}

[stevengill]

Ooo good point! We should update our memory install store and examples to this!

[seratch]

@stevengill Also, as the DevDatabase interface does not keep all the installations for bot, it does not support incoming webhooks in a good way. With the current implementation, an app can keep only the latest one in database. The developers of the app can still get other URLs in the Slack app admin pages, though.

Python/Java's built-in installation stores support historical data as the default way to cover any use cases including webhooks (custom implementations do not need to have historical data). Here are examples: Python SQLAlchemy, Java S3

The DevDatabase is not production-ready. That being said, the reference implementation should properly support incoming webhooks too.