Enable a setting for Codecov dedicated cloud customers, who would like to use Codecov <> Sentry Integration.

### Problem Statement

**What product do you want to improve?**
Codecov <> Sentry integration as outlined here:
https://www.notion.so/sentry/Codecov-Integration-ed7279cade5d4de08711abf30a8a5949#1e50bb0cc56f498bad250510c46e5af0

**Is your feature request related to a problem? Please describe.**
Dedicated cloud customers, which have a custom URL (i.e. `https://custom.codecov.io/url`) cannot leverage the aforementioned integration.

**Describe the solution you'd like**
Allow support for custom URL's in the Sentry settings, such as here:
https://github.com/getsentry/sentry/blob/master/src/sentry/integrations/utils/codecov.py#L16
Here we would replace `https://api.codecov.io/api/v2/` with `https://api-customurl.codecov.io/api/v2/`, based on the API URL of a given customer instance. Ideally this URL should be configurable in the app UI.

### Solution Brainstorm

Replace hard-coded URL's with variables, which can be configured in the application/settings.

### Product Area

User Feedback

Start with src/sentry/integrations/utils/codecov.py and review how the Sentry integration currently constructs the Codecov API URL. Trace the relevant Sentry settings or configuration entry points and determine how a dedicated customer's API URL can be supplied. Done means the integration supports the custom URL described in the issue and remains usable with the default Codecov URL.

Written by the indexing model from the issue text.