Added ability to retrieve and manage Channels by key

You can now retrieve, update, and delete Channels by user-defined key, and check for their existence by key.

Changes:

• [API] Added Get Channel by Key to Channels.
• [API] Added Check if Channel exists by Key to Channels.
• [API] Added Update Channel by Key to Channels.
• [API] Added Delete Channel by Key to Channels.
• [GraphQL API] Added the key field to the updateChannel and deleteChannel fields of the Mutation type.

The following changes were introduced in terms of GraphQL SDL:

type Mutation {
  updateChannel(
    version: Long!
    actions: [ChannelUpdateAction!]!

    "Queries with specified ID"
    id: String

    "Queries with specified key"
    key: String
  ): Channel

  deleteChannel(
    version: Long!

    "Queries with specified ID"
    id: String

    "Queries with specified key"
    key: String
  ): Channel
}