Enums support #2: provide enums via FunctionNamespaceManagers

[daniel-ohayon]

This PR follows #14728
Commits start from "provide types from FunctionNamespaceMgr" (seems like we can't do stacked PRs?)

Design

We want plugins to be able to provide enum definitions. However, the current getTypes method on the Plugin interface is not sufficient, because these types are only loaded at server startup time. Instead, if people modify existing enums, or register new ones, we want those to be usable in queries immediately, without the need to reset the cluster.

These requirements are similar to those of user-defined SQL functions, which is why it makes sense to expose these types via the FunctionNamespaceManager (FNM) interface. Another argument in favor of doing this is that if users want to register UDFs that use these custom types in their signature, we don't end up with dependencies between different plugins/FNMs: the same FNM can provide the enums and the functions.

Implementation

The gist of the change is in TypeRegistry::getType, where we look up a type signature in the available FNMs if it can't be found in our built-in list of types.
The rest of the code is refactoring the plumbing to load FNMs inside a TypeRegistry.