Add ReadOnlySpan<byte> api to JsonElement.EnumerateObject()

[ghord]

Currently it is not possible to enumerate property names from JsonElement as raw spans.

There are api's that let you get property once you know the name such as JsonElement.TryGetProperty(ReadOnlySpan<byte>, out JsonElement), but the result of object enumeration - JsonProperty has only one string property Name.

Using JsonProperty.Name converts utf8 data and allocates in getter. It should be possible to get property name as a raw ReadOnlySpan<byte>.

[ghord]

It is also not possible to get raw string value as ReadOnlySpan<byte> from JsonElement that is a string. GetRawText() doesn't help - it returns string.

[ericstj]

@ahsonkhan / @steveharter / @bartonjs if they know of a way to do this using current API. If using the reader I imagine you could get at ValueSequence/ValueSpan when reading a JsonTokenType.PropertyName. I don't see a great way get at the raw property Spans when using JsonElement.

[bartonjs]

JsonElement is intentionally hiding the encoding in this release so we don't box ourselves out of supporting a UTF-16 (string) based reader in the future.

If you're wanting an equality check, we have that with JsonProperty.NameEquals (or a value check is JsonElement.ValueEquals). Anything more than that requires transcoding to System.String.

Duplicate of #30046.