Added default deploy callback + new Flow input controls

[jongpie]

Closes #5 with a few changes

• By default, a new inner class CustomMetadataSaver.DefaultDeployCallback is used (extends Metadata.DeployCallback)
  • This class has 2 boolean controls: 1 for sending an email on error, and another for on success
• Flows can now specify true/false for sending emails, as well as the name of another callback class to use (instead of the default)
  • This implementation assumes that the custom callback class has a parameterless constructor
• Apex developers can still provide their own callback class using deploy(List<SObject> customMetadataRecords, Metadata.DeployCallback callback)
• All deploy methods now return the deployment ID

[jongpie]

Screenshot of Flow options

[jongpie]

Just to document some details before merging - the default callback sends an email to the user after the deployment finishes

• Subject: Sandbox: Custom metadata type deployment completed
• Email Body:

  Deployment ID 0Af0x00000nWhKRCA0 completed
  Status: Succeeded
  1 total items deployed
  0 items failed
  Details: {
  "componentSuccesses" : [ {
      "success" : true,
      "problemType" : null,
      "problem" : null,
      "lineNumber" : null,
      "id" : "m000x000000H4rLAAS",
      "fullName" : "CustomMetadataDeployTest.hello",
      "fileName" : "customMetadata/CustomMetadataDeployTest.hello.md",
      "deleted" : false,
      "createdDate" : "2021-03-30T23:39:00.034Z",
      "created" : true,
      "componentType" : "CustomMetadata",
      "columnNumber" : null,
      "changed" : true
  }, {
      "success" : true,
      "problemType" : null,
      "problem" : null,
      "lineNumber" : null,
      "id" : null,
      "fullName" : "package.xml",
      "fileName" : "package.xml",
      "deleted" : false,
      "createdDate" : "2021-03-30T23:39:00.052Z",
      "created" : false,
      "componentType" : null,
      "columnNumber" : null,
      "changed" : true
  } ],
  "componentFailures" : [ ]
  }

• Screenshot