feat: support configurable chunk size

[dominiqueclarke]

Adds an environment variable to configure chunk size when pushing monitors to Kibana.

Testing

1. Create a test project
2. Check out this PR. Run npm install then npm run build.
3. Run npm link
4. Check out the directory for the test project
5. run npm link @elastic/synthetics
6. Run CHUNK_SIZE=1 SYNTHETICS_API_KEY=MXhmQURwRUI1UGtRakt5TW5zWk46VWtFTWJrbE9RMjZTdFBrRVpFd190Zw== npm run push
7. Watch as only 1 monitor is pushed at a time

[vigneshshanmugam]

LGTM

[jasonrhodes]

Approving on behalf of our team but because Vignesh already reviewed and approved. We will fix this repo so his approval is enough (but keep our team as possible reviewers too, even if we have to create a new GH team to achieve this)...

[vigneshshanmugam]

New changes require approval from someone other than vigneshshanmugam because they were the last pusher.

I think its because I pushed a change.

[vigneshshanmugam]

@dominiqueclarke If you can squash my commit along with yours and remove mine using rebase, We can get this merged faster.

[jasonrhodes]

@vigneshshanmugam are you the only one who can approve PRs for merge in this repo?