**Description**

Expose the connection into the jinja template context via `conn.value.<connectionname>.{host,port,login,password,extra_config,etc}`

Today is possible to conveniently access [airflow's variables](https://airflow.apache.org/docs/apache-airflow/stable/concepts.html#variables) in jinja templates using `{{ var.value.<variable_name> }}`. 

There is no equivalent (to my knowledge for [connections](https://airflow.apache.org/docs/apache-airflow/stable/concepts.html#connections)), I understand that most of the time connection are used programmatically in Operators and Hooks source code, but there are use cases where the connection info has to be pass as parameters to the operators and then it becomes cumbersome to do it without jinja template syntax.

I seen workarounds like using [user defined macros to provide get_login(my_conn_id)](https://stackoverflow.com/questions/65826404/use-airflow-connection-from-a-jinja-template/65873023#65873023
), but I'm after a consistent interface for accessing both variables and connections in the same way


**Workaround**
The following `user_defined_macro` (from my [stackoverflow answer](https://stackoverflow.com/a/66471911/90580)) provides the suggested syntax `connection.mssql.host` where `mssql` is the connection name: 

```
   class ConnectionGrabber:
       def __getattr__(self, name):
           return  Connection.get_connection_from_secrets(name)
dag = DAG( user_defined_macros={'connection': ConnectionGrabber()}, ...)
 task = BashOperator(task_id='read_connection', bash_command='echo {{connection.mssql.host }}', dag=dag)
```

This macro can be added to each DAG individually or to all DAGs via an Airflow's Plugin. What I suggest is to make this macro part of the default. 

**Use case / motivation**

For example, passing credentials to a [KubernetesPodOperator](https://airflow.apache.org/docs/apache-airflow-providers-cncf-kubernetes/stable/operators.html#howto-operator-kubernetespodoperator) via [env_vars](https://cloud.google.com/composer/docs/how-to/using/using-kubernetes-pod-operator) today has to be done like this: 

```
connection = Connection.get_connection_from_secrets('somecredentials')
k = KubernetesPodOperator(
  task_id='task1', 
  env_vars={'MY_VALUE': '{{ var.value.my_value }}', 'PWD': conn.password,},
  )
```
where I would prefer to use consistent syntax for both variables and connections like this: 
```
# not needed anymore: connection = Connection.get_connection_from_secrets('somecredentials')
k = KubernetesPodOperator(
  task_id='task1', 
  env_vars={'MY_VALUE': '{{ var.value.my_value }}', 'PWD':  '{{ conn.somecredentials.password }}',},
  )
```

The same applies to `BashOperator` where I sometimes feel the need to pass connection information to the templated script.

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

yes, I can write the PR.

**Related Issues**


airflow/models/taskinstance.py
self,
        task_ids: Optional[Union[str, Iterable[str]]] = None,
        dag_id: Optional[str] = None,
        key: str = XCOM_RETURN_KEY,
        include_prior_dates: bool = False,
        session: Session = None,
    ) -> Any:
        """
        Pull XComs that optionally meet certain criteria.
        The default value for `key` limits the search to XComs
        that were returned by other tasks (as opposed to those that were pushed
        manually). To remove this filter, pass key=None (or any desired value).
        If a single task_id string is provided, the result is the value of the
        most recent matching XCom from that task_id. If multiple task_ids are
        provided, a tuple of matching values is returned. None is returned
        whenever no matches are found.
        :param key: A key for the XCom. If provided, only XComs with matching
            keys will be returned. The default key is 'return_value', also
            available as a constant XCOM_RETURN_KEY. This key is automatically
            given to XComs returned by tasks (as opposed to being pushed
            manually). To remove the filter, pass key=None.
        :type key: str
        :param task_ids: Only XComs from tasks with matching ids will be
            pulled. Can pass None to remove the filter.
        :type task_ids: str or iterable of strings (representing task_ids)
        :param dag_id: If provided, only pulls XComs from this DAG.
            If None (default), the DAG of the calling task is used.