[Synthetics] Monitor config serialization is incorrect for inline monitors

[vigneshshanmugam]

Kibana version: main

Describe the bug:

When creating Synthetics monitors, If template strings are used in the Synthetics script steps for the Multi Step journeys, Execution of that monitor results in yaml parsing error on the Heartbeat side due to incorrect string serialization.

Steps to reproduce:

1. Create a Synthetics Multi Step monitor
2. Paste in any script containing steps that involves JS template literals. Example

   step('Go to example,com', async () => { 
   const url = "https://example.com"
   await page.goto(`${url}`);
   });

3. Execution will result in error that calls out the YML parsing issue job could not be initialized: could not parse browser config: missing field accessing 'heartbeat.monitors.0.streams.0.source.inline.script' (source:'/heartbeat/heartbeat.yml')

Expected behavior: Steps should be executed successfully without any errors.

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)

[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)