Add YAML support

[jcrist]

This adds a new msgspec.yaml submodule for encoding/decoding YAML messages using the type system provided by msgspec. This can be useful for configuration files, or for validating YAML before encoding it in some other format.

For example, in a past job I could have used this to load and validate k8s manifests from YAML, before processing and sending them to the k8s server as JSON.

This relies on the third-party PyYAML library for YAML support. A yaml extra has been added for providing this dependency.

For now only encode and decode functions have been provided; no Encoder/Decoder types. The builtin msgpack/json integration can benefit from reusing those objects, but the overhead of PyYAML and the type conversations makes the use of such types useless beyond interface compatibility. We can add them later if a user need arises.

[jcrist]

Example usage:

In [1]: import msgspec

In [2]: class Config(msgspec.Struct):
   ...:     host: str
   ...:     port: int
   ...:     threads: int = 1
   ...: 

In [3]: msg = """
   ...: host: 127.0.0.1
   ...: port: 8786
   ...: """

In [4]: config = msgspec.yaml.decode(msg, type=Config)  # load and validate yaml

In [5]: config
Out[5]: Config(host='127.0.0.1', port=8786, threads=1)

In [6]: bad_msg = """
   ...: host: 127.0.0.1
   ...: port: oops
   ...: """

In [7]: msgspec.yaml.decode(bad_msg, type=Config)
---------------------------------------------------------------------------
ValidationError                           Traceback (most recent call last)
Cell In[7], line 1
----> 1 msgspec.yaml.decode(bad_msg, type=Config)

File ~/Code/msgspec/msgspec/yaml.py:165, in decode(buf, type, dec_hook)
    163 if type is Any:
    164     return obj
--> 165 return _from_builtins(
    166     obj, type, builtin_types=(_datetime.datetime, _datetime.date), dec_hook=dec_hook
    167 )

ValidationError: Expected `int`, got `str` - at `$.port`

[jcrist]

This PR adds the code, tests, docstrings, and API docs for YAML support. Adding it to the prose docs (e.g. "Usage") will require a reorganization of the docs content, and will be done in a follow-up PR.