…on for legend statistics (#180917)

## Summary

This PR addresses phase one of
#181035.

Doesn't introduce any user facing changes. 
It starts supporting a new saved object property `legendStats` while
supporting a old `valuesInLegend` property. In this PR, `legendStats:
['values']` and `valuesInLegend:true` are treated as equal. When loading
the saved object, `valuesInLegend:true` is transformed to
`legendStats:['values']`. After loading the document, the Lens app logic
is built around the new `legendStats` property.
When user saves the saved object, we do a reverse operation- we save the
runtime state `legendStats:['values']` as `valuesInLegend: true` to
ensure backwards compatibility.


![image](https://github.com/elastic/kibana/assets/4283304/5e09b062-b4d3-424d-b8a8-79a08f4d6260)


Changes for runtime state:

- For xyCharts, the `valuesInLegend?: boolean ` property is replaced
with a more extensible `legend.legendStats?: LegendStats[]` interface

- For partition charts, the `showValuesInLegend?: boolean` property is
replaced with `legendStats?: LegendStats[]`.

after loading - in initialize function:

```ts
export function convertToRuntime(
  state: XYPersistedState,
  annotationGroups?: AnnotationGroups,
  references?: SavedObjectReference[]
) {
  const outputState = needsInjectReferences(state)
    ? injectReferences(state, annotationGroups, references)
    : state;
  if ('valuesInLegend' in outputState) {
    return convertToLegendStats(outputState);
  }
  return outputState;
}
```

before saving :

```ts
export function convertToPersistable(state: XYState) {
  const persistableState: XYPersistedState = convertToValuesInLegend(state);
  /.../
}
```

In the future the `legendStats` prop would contain also other types of
stats -see the [issue](#176583).

…ration for legend statistics (#181953)

## Summary

To the code owners,

This PR updates the saved object of Lens with the following changes: the
property `valuesInLegend: boolean` is converted to the new property
`legendStats: ["currentAndLastValue"]` (cartesian) or `legendStats:
["value"]`(partition) to accommodate displaying additional legend
statistics in the future. There are no user-facing changes introduced. I
have updated all saved object shapes in the code to adhere to the new
structure. While the old structure will continue to function properly,
this update ensures cleaner code.

Fixes #181035 (phase 2). It's is
a continuation of #180917.

In comparison to the phase 1, we've removed the conversion of
legendStats to valuesInLegend which was initially added in the previous
PR to ensure backward compatibility for serverless applications.

<img width="807" alt="Screenshot 2024-04-25 at 10 51 50"
src="https://github.com/elastic/kibana/assets/4283304/6c23d035-f64d-4fb1-96c6-d30392ae6752">

I won't merge till the end of the week as it has to be released
separately from phase 1.

---------

Co-authored-by: Cauê Marcondes <55978943+cauemarcondes@users.noreply.github.com>
Co-authored-by: Alex Szabo <alex.szabo@elastic.co>

…ration for legend statistics (elastic#181953)

## Summary

To the code owners,

This PR updates the saved object of Lens with the following changes: the
property `valuesInLegend: boolean` is converted to the new property
`legendStats: ["currentAndLastValue"]` (cartesian) or `legendStats:
["value"]`(partition) to accommodate displaying additional legend
statistics in the future. There are no user-facing changes introduced. I
have updated all saved object shapes in the code to adhere to the new
structure. While the old structure will continue to function properly,
this update ensures cleaner code.

Fixes elastic#181035 (phase 2). It's is
a continuation of elastic#180917.

In comparison to the phase 1, we've removed the conversion of
legendStats to valuesInLegend which was initially added in the previous
PR to ensure backward compatibility for serverless applications.

<img width="807" alt="Screenshot 2024-04-25 at 10 51 50"
src="https://github.com/elastic/kibana/assets/4283304/6c23d035-f64d-4fb1-96c6-d30392ae6752">

I won't merge till the end of the week as it has to be released
separately from phase 1.

---------

Co-authored-by: Cauê Marcondes <55978943+cauemarcondes@users.noreply.github.com>
Co-authored-by: Alex Szabo <alex.szabo@elastic.co>