Add JSON and Protobuf native schema support

[Thorbenl]

Fixes #254

Description

Implements ISchema<T> for JSON and Protobuf message types, bringing DotPulsar closer to feature parity with the Java and Go Pulsar clients.

New schemas:

• JsonSchema<T>: Serializes/deserializes using System.Text.Json. Supports optional JsonSerializerOptions for customization and an optional JSON schema definition string for broker-side schema compatibility checking. Uses SchemaType.Json.
• ProtobufSchema<T>: Constrained to Google.Protobuf.IMessage<T>. Uses native protobuf serialization with SchemaType.ProtobufNative (type 20). Extracts the file descriptor from the message type as schema data. No new dependencies — Google.Protobuf is already a project dependency.

Other changes:

• Schema.Json<T>() and Schema.Protobuf<T>() factory methods added to Schema.cs, following the existing pattern for AvroISpecificRecord<T>() and AvroGenericRecord<T>().
• Added inner exception constructor to SchemaSerializationException to properly wrap serialization errors (matching SchemaException which already has this overload).
• Added System.Text.Json package reference for netstandard2.0 and netstandard2.1 targets only (already part of the BCL for net8.0+).

Design decisions worth discussing:

1. ProtobufNative (type 20) vs Protobuf (type 3): We chose ProtobufNative which uses the native protobuf file descriptor as schema data. Type 3 uses Avro-encoded schema representation which would require an Avro dependency. Happy to adjust if maintainers prefer type 3.
2. JSON schema data: The default constructor sends empty schema data (matching how primitive schemas like BooleanSchema work). An overload accepting a schema definition string is available for broker-side schema evolution checking.
3. IMessage<T> disambiguation: Google.Protobuf.IMessage<T> is fully qualified in the constraint to avoid ambiguity with DotPulsar.Abstractions.IMessage<T>.

Regression

No. This is a new feature adding two new schema types. No existing functionality is modified beyond adding an inner exception constructor to SchemaSerializationException.

Testing

• 12 unit tests for JsonSchema<T> covering: construction, schema info, encode, decode, round-trip, custom JsonSerializerOptions (camelCase), schema definition passthrough, and empty bytes error handling.
• 9 unit tests for ProtobufSchema<T> covering: construction, schema info (type, name, data), encode, decode, round-trip with StringValue and Int32Value, empty message, and empty bytes.
• All 21 new tests pass.
• All 224 existing unit tests still pass (1 pre-existing ZstdCompressionTests failure unrelated to this change).
• Solution builds with 0 warnings, 0 errors across all 5 target frameworks (netstandard2.0, netstandard2.1, net8.0, net9.0, net10.0).

[blankensteiner]

Thanks for the PR :-)