Move `SecretsBackend` into a shared library

[amoghrajesh]

Body

SecretsBackend is used by task SDK + airflow core and would be a good candidate to move into the shared library.

Ref: https://github.com/apache/airflow/tree/main/airflow-core/src/airflow/secrets

Committer

• I acknowledge that I am a maintainer/committer of the Apache Airflow project.

[amoghrajesh]

Self assigning and starting work on this one.