Add XCom serializer for pendulum.date.Date

[mrlimacz]

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This PR implements an XCom serializer for pendulum.date.Date.

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[SameerMesiah97]

Looks good. But may I know what motivated this PR?

[mrlimacz]

@SameerMesiah97 we want to have a task used across multiple DAGs that would take dates or date times object from DAG parameters for manual runs (ad-hoc client requests, for scheduled DAGs we use data_interval_start and data_interval_end). The resulting period is returned in XComs. For convinience I use pendulum for all date(time) related operations and stumbled upon pendulum.Date not being supported (while datetime.date is).

[amoghrajesh]

@mrlimacz we do have support for pendulum.DateTime, does that not suffice your need?

Example:

>>> import pendulum
>>> s = pendulum.datetime(2021, 1, 1, tz="UTC")
>>> from airflow.sdk.serde import serialize, deserialize
>>> serialize(s)
{'__classname__': 'pendulum.datetime.DateTime', '__version__': 2, '__data__': {'timestamp': 1609459200.0, 'tz': {'__classname__': 'builtins.tuple', '__version__': 1, '__data__': ['UTC', 'pendulum.tz.timezone.Timezone', 1, True]}}}
>>> deserialize(serialize(s))
DateTime(2021, 1, 1, 0, 0, 0, tzinfo=Timezone('UTC'))
>>> deserialize(serialize(s)) == s
True

[mrlimacz]

@amoghrajesh I'd like to have the object type consistent with the input format. Whenever DAG parameters of date-time format, I'd like to return pendulum.DateTime. But when they are of format date, I'd like the serialized output to be pendulum.Date