Avoid change attributes into the constructor in Apache Beam operators

[Taragolis]

Avoid operation (assign, checks, cast) on potential templated fields into the Apache Beam Operators, all logic moved into the execute method
Avoid use mutable in the session scope into the tests.
Avoid create operators before test cases

In addition I install locally pytest-random-order and run for about 20 times pytest tests/providers/apache/beam/operators/test_beam.py --random-order without any side effect

Operators still have a lot of logic which assign a lot of attributes into the runtime in specific order.
BeamDataflowMixin looks useless because it only provided private methods and assign to the all operators.
But maybe this should be resolved as a follow up.

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[potiuk]

NICE!