keymgr: API updates and other improvements

gabi-250requested to merge
gabi-250/arti:keymgr-api-updates into main

Note: this builds on top of the changes from !1418 (closed). This branch starts at "keymgr: Add a type alias for Box<dyn Keystore>"

This contains some KeyMgr API changes needed for #862 (closed) and #894 (closed). In particular, I've given KeyMgr::get, KeyMgr::insert, KeyMgr::remove an additional param for specifying what Keystore to query/apply changes to, and clarified the semantics of these functions.

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