fix: pydantic dataclass can inherit from stdlib dataclass and arbitrary_types_allowed is supported (#2051)

* fix: pydantic dataclasses can inherit from stdlib dataclasses

closes #2042

* docs: add some documentation

* fix: support arbitrary_types_allowed with stdlib dataclass

closes #2054

* docs: add documentation for custom types

docs/examples/dataclasses_stdlib_inheritance.py

@@ -0,0 +1,27 @@
+import dataclasses
+
+import pydantic
+
+
+@dataclasses.dataclass
+class Z:
+    z: int
+
+
+@dataclasses.dataclass
+class Y(Z):
+    y: int = 0
+
+
+@pydantic.dataclasses.dataclass
+class X(Y):
+    x: int = 0
+
+
+foo = X(x=b'1', y='2', z='3')
+print(foo)
+
+try:
+    X(z='pika')
+except pydantic.ValidationError as e:
+    print(e)