rclc_executor: improve enum type names (backport #379)

[mergify]

This is an automatic backport of pull request #379 done by Mergify.
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Changes to be committed:
	modified:   rclc/src/rclc/executor.c
	modified:   rclc/test/rclc/test_executor.cpp

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	both modified:   rclc/include/rclc/executor.h

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[JanStaschulat]

no backport on humble, because additional data structures were only added in Iron and Rolling.
TODO:

• - update documentation for rclc_executor_semantics_t
• - improve type names for rclc_executor_semantics_t