[Maps][File upload] Add buffer to chunk size to prevent risk of oversize post requests

[kindsun]

Summary

Resolves #45510. The logic currently used to chunk up a FeatureCollection across several posts on the client uses the same limit constant MAX_BYTES used on the server as a POST request size limit. The problem that occasionally arises is when an array of features assigned to a chunk come so close to the MAX_BYTES limit, that the remaining POST content that's added is enough to push the overall POST over the size limit. This PR just subtracts a 2MB buffer from the MAX_BYTES size to create a maxByteSize chunk limit that allows room for remaining POST content to be added without exceeding the server's limits.

Steps to test locally

1. Unzip the attached file to get the GeoJSON file used in the original issue
2. Upload the GeoJSON File using the Maps app

Note: With this particular file there are several features which fail to index due to Elasticsearch errors, this is not related to this issue.

buildings-46900-mini.geo.json.tar.gz

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: e2c18e5

[kindsun]

@elasticmachine merge upstream

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 3a844e8

[kindsun]

@elasticmachine merge upstream

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 0714c65

[thomasneirynck]

thanks, nice layer btw.

I'd not backport to 7.5, but it's a small enough change. up2you.