Add support for layered and plugin configurations (#1543)

- This adds support for reading configurations from
  a three layer hierarchy:
  1. User provided TOML
  2. Install root TOML
  3. Code defaults
- We add some code to support plugin configurations
  via the ConfigurationLoader. Each plugin can provide
  a struct with an accompanying id that gets used to
  parse the scoped section of the TOML.

Sources/ContainerPersistence/ConfigSnapshotDecoder.swift

@@ -61,6 +61,16 @@ public struct ConfigSnapshotDecoder: Sendable {
         _ type: T.Type,
         from snapshot: ConfigSnapshotReader
     ) throws -> T {
+        if type is any UnsupportedDictionaryDecoding.Type {
+            throw DecodingError.typeMismatch(
+                T.self,
+                DecodingError.Context(
+                    codingPath: [],
+                    debugDescription:
+                        "ConfigSnapshotDecoder does not support decoding dictionaries (got \(T.self)). Represent dynamic keys as nested structs with known property names."
+                )
+            )
+        }
         let decoder = ConfigSnapshotDecoderImpl(
             snapshot: snapshot,
             codingPath: [],