No exported setter for the package-level ProblemConfig #15

GetProblemTypeURL reads from a package-level defaultProblemConfig (built once via NewProblemConfig()), but there's no exported way to replace it — only DefaultProblemConfig() (read-only copy) and the per-call ParseOptions.Problems override on ParseRequest.

Consumers that want a custom BaseURL (to emit real RFC 9457 type URIs instead of bare words like "unauthorized") or additional ErrorTypePaths globally have no way to configure this once at startup — they'd have to pass ParseOptions.Problems on every single ParseRequest call, and there's no equivalent hook at all for the direct helpers (NewProblemDetails, ProblemBadRequest, etc.), which always resolve via the package-level default.

Found while auditing usage across a ~150k LOC service (GolgiMed): 60+ call sites construct ProblemDetails with a bare-word type ("unauthorized", "forbidden", ...) instead of a proper URI, because there's no way to register a base URL once and have every helper pick it up.

Requested: SetProblemConfig(cfg ProblemConfig) (mirroring SetValidator/ClearValidator's pattern) so an app can configure BaseURL + ErrorTypePaths once at composition root.