Add an extra_constant to pallet-treasury (#7918)

Aligned with #7206, this PR exposes `pallet-treasury`'s pot account as
an extra constant. Users can this way for instance donate to the chain's
treasury account, or founders can easily spot it in case they require to
fund it.

In any case this greatly reduces errors and headaches trying to figure
out what's the pot's address.

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Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>