[IMP] commission: allow to set specific commissions for partners

[odooNextev]

Is your feature request related to a problem?
An agent can have a different commission based on the partner, but now commission field is only set on the agent itself.

Describe the solution you'd like
An intermediate model between res.partner and commission could be created and it could be called something like partner.agent.commission, then a one2many field agent_line_ids can replace agent_ids.

By default, adding a line in partner.agent.commission field and setting an agent, will populate commission field with value set in agent commission field, then you can change it as you want.