Merge all configuration

[DiamondJoseph]

Parked while I return to DLS work.

• Authenticator, AccessPolicy, Tree deserialization modified to behave like a discriminated union
• Impls of AccessPolicy directly extend it for type checking purposes
• Authentication top level of config merged into Settings object
• Tree -> Trees in Settings object* [not sure about this- presumably there is a MapAdapter type that can hold a list of MapAdapters and be a root Tree instead?]
• Authenticator.providers + Authenticator -> Authenticators in Settings object (until can reduce down to 1)
• AccessPolicy on root of Settings documented* [not sure about this- re: Tree above, if root Tree object can put access policy on that]
• AccessPolicy for individual trees moved out of MapAdapter object* [not sure about this- do we need on MapAdapter? Or just pass in the Tree + AccessPolicy + Path into functions that check access]

Checklist

• [ ] Add a Changelog entry
• [ ] Add the ticket number which this PR closes to the comment section