Add support for recursive messages (#130)

Changes message initialization (`__post_init__`) so that default values
are no longer eagerly created to prevent infinite recursion when
initializing recursive messages.

As a result, `PLACEHOLDER` will be present in the message for any
uninitialized fields.  So, an implementation of `__get_attribute__` is
added that checks for `PLACEHOLDER` and lazily creates and stores
default field values.

And, because `PLACEHOLDER` values don't compare equal with zero values,
a custom implementation of `__eq__` is provided, and the code generation
template is updated so that messages generate with `@dataclass(eq=False)`.

Also add new Message __repr__ implementation that skips PLACEHOLDER 
values and orders keys by number from the proto.

Co-authored-by: Christopher Chambers <chris@peanutcode.com>
Co-authored-by: nat <n@natn.me>
Co-authored-by: James <50501825+Gobot1234@users.noreply.github.com>

src/betterproto/templates/template.py.j2

@@ -37,7 +37,7 @@ class {{ enum.py_name }}(betterproto.Enum):
 {% endfor %}
 {% endif %}
 {% for message in output_file.messages %}
-@dataclass
+@dataclass(eq=False, repr=False)
 class {{ message.py_name }}(betterproto.Message):
     {% if message.comment %}
 {{ message.comment }}