Add OIDCProviderRecord — the persisted, IAM-managed view of an OIDC
identity provider — and an OIDCProviderStore interface with memory and
filer implementations mirroring the existing role-store pattern.
The store is hydrated at boot from the static STS.Providers list so the
new IAM API surfaces the same set the STS service already validates
against. Two read-only actions land now:
- ListOpenIDConnectProviders -> ARN-only list, AWS-shape XML.
- GetOpenIDConnectProvider -> URL, ClientIDList, ThumbprintList,
Tags, CreateDate.
Mutations (Create/Delete/Add-Remove ClientID/Update Thumbprint), multiple
client_ids per provider, and TLS thumbprint pinning come in Phase 2b.
