Reported by @javanna for the 7.17 Java client: the index.mapping.total_fields.limit index setting is missing.

Digging further, it seems most (it not all) of the index mapping settings are missing from the spec.

We should also double check all the "settings in other index modules" section of index settings.

Discussing this issue with @javanna, a few things came up:

• the full list of index mapping settings is defined statically in the ES code base. An exhaustive list of settings and they type (or even the IndexSettings TS type) could be generated from that code. This can be a build-time thing and doesn't have to be a new ES API.
• the IndexSettings type need an "escape hatch" so that users can add settings that would be missing from the API spec. Having IndexSettings implements AdditionalProperties<string, UserDefinedValue> should be enough.