[ADD] Add partner contract anniversary date fields

[samirGuesmi]

This addon add a field for first contract line start date and compute contract anniversary date, useful for renewals and customer relationship management.

[samirGuesmi]

@sbejaoui

[qgroulard]

@samirGuesmi Let's finalize this during OCA days :rocket:

[samirGuesmi]

Thanks for the review, here we are.