[Multi-provider] Reconsider type of "originalMetadata"

For now I kept originalMetadata as Map to match how buildProviders currently treats provider names as keys.
I agree a List> could better represent duplicate names; happy to open a follow-up issue to discuss that representation and adjust the metadata type there if we decide to support duplicates fully.

_Originally posted by @Vishalup29 in https://github.com/open-feature/java-sdk/pull/1765#discussion_r2633568350_