Pass data about the internal DB connection to the front end via common_data

[seancolsen]

Mathesar stores its metadata within a Django "internal database". The connection to this database is defined via environment variables, and it can either be a PostgreSQL connection or a file path to a SQLite database.

In the "New DB Connection" form, the front end needs to know some details about this internal database connection in order to render the form.

The backend should put this internal_database field into the "common_data" bundle injected into the front end. As such, we should not need any API modifications to pass this data to the front end.

When using SQLite, the backend should pass this data to the front end:

{
  // other common_data properties here
  // ...
  "internal_database": {
    "type": "sqlite"
  }
}

When using Postgres, the backend should pass data like this:

{
  // other common_data properties here
  // ...
  "internal_database": {
    "type": "postgres",
    "user": "mathesar",
    "host": "localhost",
    "port": "5432",
    "database": "mathesar_django"
  }
}

When "type" is "postgres", all remaining properties as exemplified above must be present and must be strings (i.e. not null).