docs: Document inherited PluginBase attributes and methods

[edgarrmondragon]

Some attributes and methods, such as Tap.print_version, weren't documented because we exclude PluginBase from the docs (probably with good reason, since developers should seldom need to subclass it directly).

Now they're documented: https://meltano-sdk--1243.org.readthedocs.build/en/1243/classes/singer_sdk.Tap.html#singer_sdk.Tap.print_version

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📚 Documentation preview 📚: https://meltano-sdk--1243.org.readthedocs.build/en/1243/

[codecov]

Codecov Report

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The diff coverage is n/a.

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Impacted Files	Coverage Δ
singer_sdk/plugin_base.py	73.98% <ø> (ø)

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