[Python] Remove duplicate definition of Endpoint class

[therve]

THis moves the Endpoint class which is in every API modules to the client module where it can be imported.

PR checklist

• Read the contribution guidelines.
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cc @taxpon @frol @mbohlool @cbornet @kenjones-cisco @tomplus @Jyhess @arun-nalla @spacether

[spacether]

Thank you for the PR!