Add pre/post execution hooks (apache#17576)

Adds overrideable pre-/ post- execution hooks. With this change you can override pre-/post- hooks at the time of DAG creation, without the need of creating your own derived operators. This means that you can - for example - skip /fail any task by raising appropriate exception in a method passed as the pre- execution hook based on some criteria (for example you can make a number of tasks always skipped in a development environment). You can also plug-in post-execution behaviour this way that will be always executed at the same worker as the task run, sequentially to the task (as opposed to callbacks, which can be executed elsewhere and asynchronously)

Apache Airflow 2.1.3

Apache Airflow 2.1.3 Constraints files

Apache Airflow 2.1.3rc1

Apache Airflow constraints 2.1.3rc1

Prepares release for Salesforce provider (apache#17272)

By mistake, Salesforce provider has been marked as doc-only where it
had new connection type added. This PR updates Salesforce docs preparing
it for separate release.

Prepares release for Salesforce provider (apache#17272)

By mistake, Salesforce provider has been marked as doc-only where it
had new connection type added. This PR updates Salesforce docs preparing
it for separate release.

Prepares docs for Rc2 release of July providers (apache#17116)

Prepares docs for Rc2 release of July providers (apache#17116)

Prepares docs for Rc2 release of July providers (apache#17116)