PubSub: Make synth operations on PubSub types idempotent

[plamut]

This commit assures that the code block for injecting FlowControl docs is not added on every synth tool run - it is only added if one does not exist yet.

Contrary to my initial assumption in #8293, it appears that synthtool can be run multiple times on types.py and other files after generating them, thus adjustment is necessary.

How to test

Steps to perform:

• Install the synthtool and run it from inside the pubsub/ directory,
• Run the synthtool a few more times.

Actual result (before the fix):
A new code block for injecting FlowControl docstrings is added after every synthtool run, growing the file indefinitely.

Expected result (after the fix):
Only a single instance of the docstring-injecting code remains in the file even after multiple runs. Nothing else is changed.

[plamut]

Closing in favor of #8448 that also includes the changes from here.