Add @Required annotation

[fstjohn11]

Adds a new @Required annotation to the model package for use in marking attributes in model classes as "required". This is so we can annotate attributes of our model classes that are expected to be present on every instance of the object, so developers can safely access without any explicit null checks. Meant to be a drop-in replacement for in-line code comments or javadocs that state the same (but apply no actual enforcement at runtime)

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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[salesforce-cla]

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