• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
Adds support for webhooks as a notification channel, available as an option for escalation policy steps. Webhooks are only available to the escalation policies they were created in.

Which issue(s) this PR fixes:
Closes #2691

Out of Scope:
Future: look into better ways of differentiating webhook names as chips displayed in the UI

Screenshots:

Describe any introduced API changes:
Adds new queries for filtering and searching for webhooks.

type Webhook {
  id: String!
  name: String!
}

input WebhookSearchOptions {
  first: Int = 15
  after: String = ""
  search: String = ""
  omit: [ID!]
  escalationPolicyID: ID
}

type WebhookConnection {
  nodes: [Webhook!]!
  pageInfo: PageInfo!
}

# Allows searching for webhooks.
webhooks(input: WebhookSearchOptions): WebhookConnection!

# Returns a single webhook with the given URL.
webhook(id: String!): Webhook