**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**


tests/models/test_taskinstance.py
from airflow.models import (
    DAG,
    DagRun,
    Pool,
    RenderedTaskInstanceFields,
    TaskInstance as TI,
    TaskReschedule,
    Variable,
)
from airflow.operators.bash import BashOperator
from airflow.operators.dummy import DummyOperator
from airflow.operators.python import PythonOperator
from airflow.sensors.base import BaseSensorOperator
from airflow.sensors.python import PythonSensor
from airflow.serialization.serialized_objects import SerializedBaseOperator
from airflow.stats import Stats
from airflow.ti_deps.dependencies_deps import REQUEUEABLE_DEPS, RUNNING_DEPS
from airflow.ti_deps.dependencies_states import RUNNABLE_STATES
from airflow.ti_deps.deps.base_ti_dep import TIDepStatus
from airflow.ti_deps.deps.trigger_rule_dep import TriggerRuleDep
from airflow.utils import timezone
from airflow.utils.session import create_session, provide_session
from airflow.utils.state import State
from airflow.utils.types import DagRunType
from airflow.version import version
from tests.models import DEFAULT_DATE
from tests.test_utils import db
from tests.test_utils.asserts import assert_queries_count
from tests.test_utils.config import conf_vars
class CallbackWrapper: