Support for enums which serialize as a numeric value

Nobody has claimed this yet.

Right now, most enums are treated as strings when serialized where the name of members is the string that is used in the JSON.

I'm looking at `GeoTilePrecision` which right now is defined as `export type GeoTilePrecision = number`. In the .NET client, we [define this](https://github.com/elastic/elasticsearch-net/blob/7.x/src/Nest/Aggregations/Bucket/GeoTileGrid/GeoTilePrecision.cs) as an enum since the range is limited 0-29 and most values have a specific meaning which we include in the IntelliSense documentation e.g.
```c#
public enum GeoTilePrecision
{
///
/// Whole world
///
Precision0 = 0,
Precision1 = 1,
///
/// Subcontinental area
///
Precision2 = 2,
...
}
```
This provides a way to help consumers choose a valid value, while also providing a descriptive tooltip for values where possible.

I'd like to encode this in the spec, but I believe right now, it would assume the value to be serialised to/from a string. It's a similar story for `GeoHashPrecision`.

Start by reading the linked elasticsearch-net file src/Nest/Aggregations/Bucket/GeoTileGrid/GeoTilePrecision.cs and compare it with the specification's current enum serialization behavior. Trace how GeoTilePrecision and GeoHashPrecision would be represented, then define the spec behavior for numeric enum serialization and verify it against the affected generated clients.

Written by the indexing model from the issue text.