Use timeout instead of execution_timeout with ExternalTaskSensor when sensing task_id

[rajaths010494]

When an external task id is given ExternalTaskSensor is using execution_timeout instead of timeout from BaseSensorOperator.

closes #857

[codecov]

Codecov Report

Base: 98.63% // Head: 98.63% // No change to project coverage 👍

Coverage data is based on head (922a6b9) compared to base (fc5b37a).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #858   +/-   ##
=======================================
  Coverage   98.63%   98.63%           
=======================================
  Files          89       89           
  Lines        4974     4974           
=======================================
  Hits         4906     4906           
  Misses         68       68           

Impacted Files	Coverage Δ
astronomer/providers/core/sensors/external_task.py	95.55% <ø> (ø)

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