[Lens] Use index pattern service instead saved object client

[flash1293]

In https://github.com/flash1293/kibana/blob/7e533f26aa0f0ca6029834e53ab00fdb0c590f15/x-pack/plugins/lens/public/indexpattern_datasource/loader.ts#L46 Lens is using the saved object client directly to fetch index patterns instead of using the indexPatterns service provided by the data plugin.

This means Lens is missing some integrations the index pattern service handles (e.g. caching and dotted field names), and won't integrate without custom work with changes like #73021 (comment)

By switching to the index pattern service to fetch index patterns inside Lens, we can simplify some aspects of the loading. The change can very likely be contained within the loader function.

Part of this PR should also be to use the displayName property of fields instead of the raw field name (#73306)

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

Let's make sure to consider how Lens is handling rollup information: we have a structure called aggregationRestrictions which is not the same as the rollup information in the index pattern service.