feat: add pydantic custom type CronType

[lucianosrp]

I added this on my project and I thought you may want to consider adding it to the core script (?)

It adds a pydantic custom type called CronType.
Which makes it easy to add a Cron object in a pydatic model like below:

>>> from pydantic import BaseModel
... from cron_converter import CronType
...
... class CustomModel(BaseModel):
...     cron: CronType
... 
... model = CustomModel(cron='* * * * *')

>>> model.cron
Cron('* * * * *')

It does not add pydanitc as a core requirement - CronType is made available only if pydantic has been already imported.

Again, not sure if you are willing to integrate this PR or if you would rather have it as a separate extentsion. I am fine either way 🤗

[lucianosrp]

I realised that Annotated requires python>=3.9

There are a couple of ways to solve that.
The best option would be to add typing-extensions for python<=3.8
What do you think?

[Sonic0]

@lucianosrp your contributions are always welcome 🤩. I'm unprepared for this topic and I need time to understand your code.
Compatibility with Python 3.8 wouldn't be a problem, just a few weeks and it will be in 'end of life' status.
I need to study something about your code, allow me some time to look into it.