Allow starting the sideloader for the tooling session

[plexus]

CIDER's deferred loading of middleware means that requires can happen during all
kinds of nREPL ops, many of which are performed over the tooling session. To
make this work via the sideloader we need it to be active both on the regular
session and on the tooling session, so provide a flag for starting it in either
session.

Part of #3037

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• You've added tests (if possible) to cover your change(s)
• All tests are passing (eldev test)
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• You've updated the changelog (if adding/changing user-visible functionality)
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[plexus]

I originally had the op automatically go to both sessions, the more granular version seems preferable though. Not sure if it really makes sense to add tests here or doc updates.

[bbatsov]

I wonder if a more generic session parameter wouldn't be more appropriate here. Technically a connection can have other sessions than the two created by CIDER by default, and this would allow us to interact with them. I guess this will require a small change in nrepl-send-request as well, though.

[bbatsov]

Anyways, I can just do this myself down the road. Let's keep your change focused to the problem at hand.

[bbatsov]

I don't think tests/doc updates for this are needed. I agree it makes sense for the tooling session to enable sideloading as well.