Description of the issue/feature this PR addresses: Odoo has been growing and more and more technical fields have been added who either contain rather sensitive data or nothing useful for exporting. See also the discussion at https://twitter.com/ggellatly/status/1524145898380738561

Current behavior before PR: Every single field in Odoo is by default shown in the export dialog causing an overload of info with quite often content shown that has no use (or should even be hidden for semi-security reasons).

Desired behavior after PR is merged:
By adding an attribute `exportable` on the `ir.model.fields` we can custom define which fields we want to be exportable or not.
For example: by default on the `sale.order` model the field `access_token` is by default exportable. If you'd flag the field as non-exportable it would no longer show up and thus not leak (rather sensitive) data:
```
# -*- coding: utf-8 -*-

from odoo import models, fields, api


class SaleOrder(models.Model):
    _inherit = 'sale.order'

    access_token = fields.Char(
        exportable=False
    )
```

By adding this configurable flag a developer could override any field and simply add the `exportable` flag which is set to `False`.




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