fix MapsterMapper#281 null propagation should return nullable type

fix MapsterMapper#282 map error on record type with empty ctor

- Split attributes and helper classes to Mapster.Core (for codegen pr…

…oject)

- Support C# 9.0 record type
- [Codegen] Support NET 5.0
- [Codegen] Fluent API to generate mapping

fix CreateNewIfNull

fix UseDestinationValue with collection
fix MapsterMapper#272 IEnumerable is enumerating twice when adapted to array
fix MapsterMapper#269 RequireExplicitMapping should validate child mapping
fix MapsterMapper#266 ProjectToType doesn't work with DI(MapContext)

[mapster] non-generic ProjectToType

[mapster] fix MapsterMapper#263 fix when source path is Dictionary
[mapster] fix MapsterMapper#258 enforce cast projection type to prevent EF error
[codegen] generate models
[codegen] generate extension methods
[codegen] support access modifier
[sample] codegen sample

fix MapsterMapper#256 exclude backing fields from mapping

fix code generation for "include"

Mapster.Tool

Fork setting

fix race conditions on unit test (sp. thx to napalu)
fix MapsterMapper#247 Transform the same instance
fix MapsterMapper#251 Map nested IDictionary<string, object>
fix MapsterMapper#252 map same reference to different types

Fix property name match when using AdaptMember Attribute (sp. thx to …

…prgood)

close MapsterMapper#235 ISet support

fix MapsterMapper#188 Mapping to inherited interface not working