Clean up and fix DynamicValue schema handling

[vigoo]

• The implicit schema for DynamicValue is now Schema.Dynamic as it is a single placeholder when the schema is serialized instead of redundant inclusion of the whole real schema
• The real schema for DynamicValue is defined as DynamicValue.schema as with other types (DynamicValueSchema should not be private #332) and it is made public for implementing codecs outside of zio.schema
• Added some new tests that include DynamicValue in generated schemas
• Fixed the issue that MetaSchema including a Dynamic node was not serializable and tests this with JSON and Protobuf