Allow to fetch whole tree from the settings merged with the defaults
Support for a new type of API key

In order to solve the initial handshake problem with apps, OctoPrint now supports so called app session keys which are basically API keys with a limited validity. Obtaining those keys is based on a handshake procedure backed by RSA signatures. OctoPrint needs to be aware of apps and their associated public keys (with the AppPlugin there exists a mechanism to add additional recognized apps by installing a plugin). Apps perform the handshake by first requesting a temporary key with very limited validity, then sending a message back to OctoPrint containing their id, version, the temporary key and a signature created with their private key over these three pieces of data. OctoPrint then tries to verify the signature and if successful unlocks the key to be used as a fully recognized API key.