Allow field mappings on heroku connect to be read-only or writeable for a given object

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Heroku Connect

### Tell us about what you're trying to solve. What challenges are you facing?

Today, when you map a object on Heroku connect, you can make that object as writable or read only. I have a use-case where I want to be able to write to only two fields on that object and remaining should be readonly. Currently the issue I am facing is some other fields are also getting updated (this is just the timing of the update, when Heroku connect tries to sync the field and someone is actually updating it once the udpate is done, Heroku connect ends up overwriting the value to the old one). This creates a lot of confusion for the customer thinking we also touch other fields on that object.

There is no way today to mark certain fields of a mapped object as writable or readable. If this is allowed we can guarantee that no other fields will be updated than the specified ones.