task: make the handler field support only snake_case

[GrantBirki]

This issue tracks work to make the handler: <value> field of the endpoint(s) configuration files support only snake_case to match the same enforcements of the auth.type field. So instead of handler: Team1Handler it should just be handler: team_1_handler.

Another more in-depth example:

If a file is called handlers/github_handler.rb and its contents are:

# frozen_string_literal: true

# Example handler for GitHub webhooks
class GithubHandler < Hooks::Plugins::Handlers::Base
  # Process GitHub webhook
  #
  # @param payload [Hash, String] GitHub webhook payload
  # @param headers [Hash<String, String>] HTTP headers
  # @param config [Hash] Endpoint configuration
  # @return [Hash] Response data
  def call(payload:, headers:, env:, config:)
    log.info("🚀 Processing GitHub webhook 🚀")
    return {
      status: "success"
    }
  end
end

It should be referenced via handler: github_handler in the endpoint config. This is a breaking change and it should not be made backwards compatible. Moving forward, no more pascal case. Ruby Classes are pascal case but we will be referencing them via snake_case in the config files.