Move Steam Entity to separate file

[joostlek]

Proposed change

Recently I learned that base class entities should be moved to a separate file. And I noticed this wasn't the case in this integration so hereby :).

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[home-assistant]

Hey there @tkdrob, mind taking a look at this pull request as it has been labeled with an integration (steam_online) you are listed as a code owner for? Thanks!

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[epenet]

LGTM 👍

Though I'm not sure a base entity is useful here if it is only used by a single class.
A base entity is recommended when functionnality is shared between various platforms or multiple classes.

I wouldn't have recommended it here - but since it already exists I think it's OK.