[Heartbeat] Introduce new source for pushed monitors

[vigneshshanmugam]

Describe the enhancement:

• We should add the ability in HB to be able to run pushed monitors from the Synthetics agent to Kibana. This would be available as a new source in addition to the previous sources like inline, local and zip_url. The new source would be named as encoded/pushed, we can tweak the naming bits in the PR.

heartbeat.monitors:
  - type: browser
    id: my-monitor
    name: My Monitor
    source:
      pushed:
        content: // base 64 encoded content linking to the bundled source
    schedule: "@every 1m"

• Heartbeat would unpack the contents of the base 64 encoded zipped source and run the specific journey via filter_journeys flag - https://www.elastic.co/guide/en/beats/heartbeat/master/monitor-browser-options.html#monitor-browser-filter-journeys as the bundled source contains more than one journey depending on how the project is structured.

• The file structure should be mimicked as how the user would have pushed the source to Kibana to make sense of the stack trace when there are errors thrown.

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)