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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• Yes (please specify the tool below)

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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).