Allow specifying which fields to exclude from update in upsert/upsertMany functions

[squivix]

Is your feature request related to a problem? Please describe.
Currently, when using the entity manager's em.upsert or em.upsertMany functions to upsert data, all fields other than the unique ones are included in the update. However, sometimes we need to keep some of the fields as they are if the row already exists, but insert a new value for them if it doesn't.

Describe the solution you'd like
I would like to be able to specify which fields to exclude from the on conflict update clause. Maybewe can add an excludeFieldsOnUpdate option in the NativeInsertUpdateOptions which is an array of field names that should be kept as is if the row already exists, but inserted with new values if it doesn't.

The proposed option looks like:

const author = await em.upsertMany(Author, [
  { email: 'foo@bar.com', name:"Foo Bar", age: 33 },
  { email: 'lol@lol.lol', name:"John Doe", age: 666 },
], {excludeFieldsOnUpdate:["name"]});

Describe alternatives you've considered
Some alternatives are to use the query builder or a raw SQL query instead of the ORM's em.upsert or em.upsertMany functions, but it would be nice to have the option through the EntityManager.

Additional context
This is based on discussion #4317

[jsprw]

Have you worked on this yet? I'm also interested in this feature and will be able to work on this.

[squivix]

Have you worked on this yet? I'm also interested in this feature and will be able to work on this.

I 'm struggling to set up the repo and get it to run as I'm not very familiar with docker. Also currently busy with some other stuff. Feel free to work on it. I will probably try again in about a week

[B4nan]

FYI #4602 (comment)