Adding a compat shim for secrets_masker that was moved to shared

[amoghrajesh]

After the merge of #54449, we nuked the airflow.sdk.execution_time.secrets_masker path, but it is possible that some provider etc would be using this since it is public API in their code which would break.

Adding a compat shim for that so that those code paths to continue working and providing them some time to migrate till the next release probably.

Before changes:

(airflow) ➜  airflow git:(compat-module-for-secrets-masker) ✗ git checkout main                                                               
Switched to branch 'main'
Your branch is up to date with 'origin/main'.
(airflow) ➜  airflow git:(main) ✗ python           
Python 3.12.9 (main, Feb  4 2025, 14:38:38) [Clang 16.0.0 (clang-1600.0.26.6)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from airflow.sdk.execution_time.secrets_masker import Redactable, Redacted, SecretsMasker, should_hide_value_for_key
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'airflow.sdk.execution_time.secrets_masker'

After changes:

(airflow) ➜  airflow git:(compat-module-for-secrets-masker) ✗ python
Python 3.12.9 (main, Feb  4 2025, 14:38:38) [Clang 16.0.0 (clang-1600.0.26.6)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from airflow.sdk.execution_time.secrets_masker import Redactable, Redacted, SecretsMasker, should_hide_value_for_key
<stdin>:1 DeprecationWarning: Importing from 'airflow.sdk.execution_time.secrets_masker' is deprecated and will be removed in a future version. Please use 'airflow.sdk._shared.secrets_masker' instead.

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.