Add support for authentication as a GitHub App

[fatmcgav-depop]

This commit adds support for authentication as a GitHub App. This is beneficial in several ways, including:

• Increased rate limits
• Better separation of access
• Finer grained control over access
• Removes the need for a bot or service account

As part of these changes, have added the github.com/bradleyfalzon/ghinstallation/v2 module and it's associated dependencies.

Added several new configuration fields:

• UseGitHubApp - Boolean flag signalling if user wants to auth as a GitHub App
• PrivateKey - PEM encoded private key
• PrivateKeyFile - Filename for RSA Private Key generated for GitHub App
• AppID - GitHub App application numerical identifier
• InstallationID - GitHub App installation numerical identifier

N.B I'm currently working on documentation and integration testing, however I thought it would be useful to get early feedback...

[duelyy]

Any updates on when this can be merged? Would love to use Github App as an authentication to take advantage of the 15,000 requests per hour (rate limit).