server, agent: github app installation clone support

[benbroadaway]

Minimal support for cloning repos from GitHub app installation credentials (private key).

System-provided auth is defined in server and agent configs in the github.appInstallation section. See default conf for description.

User-provided app installation info (e.g. client id, private key) must be in a single-value secret containing JSON-formatted data like:

{
  "githubAppInstallation": {
    "apiUrl": "https://api.github.com",
    "clientId": "the-client-id",
    "privateKey": "-----BEGIN RSA PRIVATE KEY-----\nMIIEowI..."
    "username": "x-access-token",
    "urlPattern": "(?<baseUrl>github.com)/owner/.*"
  }
}

apiUrl and username are optional and default to https://api.github.com and x-access-token, respectively.