Custom fields on ProductVariantPrice

[HausTechTeam]

Is your feature request related to a problem? Please describe.
I would like to be able to connect other entities to ProductVariantPrice in order to implement custom pricing management. The use case is to enable customer-specific prices and campaign prices (discounted prices displayed in the product listing) via price lists, where the price list is connected to each individual variant price.

Describe the solution you'd like
Support for custom fields on ProductVariantPrice

Describe alternatives you've considered
Other entities that work separately but it seems redundant when Vendure already has support for multiple prices (in different currencies) that are also channel-aware.

[HausTechTeam]

Here is a PR with an implementation of the requested feature: #2654