Support loading from attribute-based objects (from_attributes)

[rnag]

This issue tracks a possible enhancement discussed in #185.

Idea Allow fromdict / loaders to support loading from attribute-based objects (e.g. ORM models, SimpleNamespace, or custom objects), similar to a from_attributes=True option.

Open questions

• What kinds of input objects should be supported?
• Should attribute access replace dict access, or act as a fallback?
• How should this interact with aliases and AliasPath?

Proposed direction (tentative)

• Default behavior unchanged (mapping-only).
• Optional opt-in via LoadMeta(from_attributes=True) or v1 equivalent.
• If enabled and input is not a mapping, resolve values via getattr.

Linked from discussion #185.