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 (deserialization works, but we need a generic way to set the path-parameters)
    • NDJSON requests (IStreamSerializable)
  • Responses
    • Regular responses
    • Dictionary responses
    • NDJSON responses (IStreamSerializable)
  • Regular classes
  • Unions
  • Enums
  • Internal variants
  • External variants
  • Untagged variants
  • Container variants
  • SortOptions special case
• Change descriptors to wrap the corresponding object representations in order to re-use serialization code.

Breaking user facing changes:

1. 💥 Removed Field.Format property and corresponding constructor and inferrer overloads.
   This property was not used anymore for quite some time (got replaced by the FieldAndFormat type).
2. 💥 Field/Fields static factory methods and conversion operators no longer return nullable references but throw exceptions instead (Field) if the input string/Expression/PropertyInfo argument is null.
   This especially makes implicit conversions to Field more easily usable without having to apply the null-forgiveness operator (!) all the time.
3. 💥 Removed FieldValue.IsLazyDocument, FieldValue.IsComposite and the corresponding members in the FieldValue.ValueKind enum. These values were not used anymore for quite some time.

Non-breaking user facing changes:

1. All requests do now expose read/write access to path properties (like index, id, ...) which previously could only be initialized from the constructor.

Later/bonus:

• Do not generate custom converters when property attributes are sufficient.
• Serialization AOT support.
• Get rid of ISelfTwoWaySerializable (used for source-body/array/dictionary requests/responses).
• Get rid of ISelfSerializable (used for descriptors).
• Get rid of ElasticsearchClient.SettingsTable (used for descriptors and in some other places). Replace with ContextProvider access.
• Validate required properties and nullability.
• Replace globally registered IsADictionaryConverterFactory with case-by-case converters.

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.