Vendure: Products: Add custom fields grouped by Tab

[jeancx]

Is your feature request related to a problem? Please describe.
I miss the option of adding custom fields grouped by tab, for organizational reasons, for example, I want to add product-specific Brazilian tax tags and would prefer to separate it into a tab, so as not to be messy, by many custom fields in the index tab.

Describe the solution you'd like

bootstrap({
    customTabs: {
      Product: [
       { name: 'Taxes', key: 'taxesTab' },
      ]
   },
    customFields: {
        Product: [
            { name: 'infoUrl', type: 'string', tab: 'taxesTab' },
        ],
    },
})

Additional context
If you think this would be good, and already not have any similar plans for that, i would like of doing this PR.

[michaelbromley]

This look similar to #277.

I do intend to support this, but have no immediate plans to do it before the 1.0 release, though it can be added after. In terms of an API, I would want to spend a bit more time on research and design before implementing. Your suggestion is a good start, but perhaps it can be improved even more.