Add cron job specification to a serverless function

Description

One common use-cases for a function is when someone needs to run some code periodically. Typically we accomplish that by specifying a cron job that triggers code. In GitLab you can configure a scheduled pipeline, and define a cron job specification in the UI.

Being able to define a serverless function being automatically invoked periodically might help too. See one of the use-cases https://gitlab.slack.com/archives/C0SFP840G/p1556582874239800 (GitLab internal).

Proposal

Use CronJobSource in Knative, deploy it using

functions:
  echo:
    handler: MyEcho.my_function
    source: echo-rb/
    runtime: https://gitlab.com/gitlab-org/serverless/runtimes/ruby
    description: "echo function using Ruby runtime"
    trigger:
      - every_10_minutes

events:
  every_10_minutes:
    cron: */10 * * * *

/cc @danielgruesso @williamchia @DylanGriffith