Add machine-readable and filterable contract_period property to product offerings

Suggestions

• initial_contract_period or just contract_period (optional ISO8601 period field and filter describing the initial contract period) IMPORTANT FOR DE-CIX
• renewal_contract_period (optional ISO8601 period field describing the renewal contract period)
• initial_notice_period (optional ISO8601 period fields and filter describing the notice period which needs to be adhered to for cancelling within the initial contract period)
• renewal_notice_period (optional ISO8601 period fields and filter describing the notice period which needs to be adhered to for cancelling within the renewal contract period)

Currently (informal only, no filters)

• contract_term
• notice_period