Improve serialization

[flobernd]

Changes:

• Improve custom converter performance.
  • Replace nested JsonSerializer.{Serialize|Deserialize} calls with direct converter access (JsonSerializerOptions.GetConverter -> converter.{Read|Write}) to prevent frequent creation of top-level JSON reader stacks.
  • Prefer direct ValueSpan access instead of allocating intermediate string values (e.g. when comparing property names or parsing/formatting values).
  • Use pre-encoded JsonEncodedText constants for compile-time constant literals (e.g. property names).
  • Use Utf8Parser/Utf8Formatter for primitive value conversions.
  • Envelope-stype marker types like e.g. SourceMarker<T> should only (de-)serialize the wrapped type to prevent unnecessary allocations.
• Implement round-trip support for all types (limited to all JSON serializable types).
  • Requests
    • Regular requests
    • Requests with required path-parameters.
    • NDJSON requests (IStreamSerializable).
  • Responses
    • Regular responses
    • Dictionary responses
  • Regular classes
  • Unions
  • Enums
  • Internal variants
  • External variants
  • Untagged variants
  • Container variants
  • SortOptions special case

Breaking changes:

1. None so far

Later:

• Do not generate custom converters when property attributes are sufficient
• Serialization AOT support
• Get rid of ISelfTwoWaySerializable (used for array/dictionary requests/responses)
• Get rid of ISelfSerializable (used for descriptors)

Related to:

• 8.15.6 - General API Feedback while upgrading from NEST. #8339
  All variants now have either a public string Type (internal, external, untyped) property or public VariantType/Variant properties (container). For non-exhaustive container variants like e.g. Aggregation, these properties are settable.