**Description** 

Since sensitive informations in the connection object (specifically the extras field) are now being masked based on sensitive key names, we should consider adding some common sensitive key names.

`private_key` from [ssh connection](https://airflow.apache.org/docs/apache-airflow-providers-ssh/stable/connections/ssh.html) is an examples.

**Use case / motivation**

Extras field used to be blocked out entirely before the sensitive value masking feature (#15599).

[Before in 2.0.2](https://github.com/apache/airflow/blob/2.0.2/airflow/hooks/base.py#L78
) and [after in 2.1.0](https://github.com/apache/airflow/blob/2.1.0/airflow/hooks/base.py#L78
).

Extras field containing sensitive information now shown unless the key contains sensitive names.

**Are you willing to submit a PR?** 

@ashb has expressed interest in adding this.

airflow/utils/log/secrets_masker.py
elif isinstance(item, Iterable):
                return list(self.redact(subval) for subval in item)
            else:
                return item
        except Exception as e:  
            log.warning(
                "Unable to redact %r, please report this via <https://github.com/apache/airflow/issues>. "
                "Error was: %s: %s",
                item,
                type(e).__name__,
                str(e),
            )
            return item
    
    def add_mask(self, secret: Union[str, dict, Iterable], name: str = None):
        """Add a new secret to be masked to this filter instance."""
        if isinstance(secret, dict):
            for k, v in secret.items():
                self.add_mask(v, k)
        elif isinstance(secret, str):
            if not secret:
                return
            pattern = re.escape(secret)
            if pattern not in self.patterns and (not name or should_hide_value_for_key(name)):
                self.patterns.add(pattern)
                self.replacer = re.compile('|'.join(self.patterns))
        elif isinstance(secret, collections.abc.Iterable):
            for v in secret:
                self.add_mask(v, name)