Allow providers to add default connections

### Body Currently we can add default connections only by adding entry to: https://github.com/apache/airflow/blob/00e10560983b4cff7279100b9cf5cd5dd8cdf877/airflow/utils/db.py#L116 This means only Airflow core release can add the new default connection which is not in sync with providers. Examples: https://github.com/apache/airflow/pull/31533#discussion_r1236704083 https://github.com/apache/airflow/pull/31787#discussion_r1234913952 This is a pain because providers support earlier Airflow version (for example right now the minimum Airlfow version supported by providers is Airflow 2.4.0 so even if we add new connection to core and release it with Airflow 2.7.0 the users of 2.4.0 won't have it) **The task:** We need somehow to make it so that providers can add default connections (and extract the existed default provider connections from core) ### Committer - [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.