feat(shared/auth): support software_statement in OAuthClientMetadata (#696)

Per [Section 3.1.1][ref], `software_statement` is an OPTIONAL member
of the client creation request, which may contain a JWT encoding claims
about client software.

[ref]: https://datatracker.ietf.org/doc/html/rfc7591#section-3.1.1

src/shared/auth.ts

@@ -98,6 +98,7 @@ export const OAuthClientMetadataSchema = z.object({
   jwks: z.any().optional(),
   software_id: z.string().optional(),
   software_version: z.string().optional(),
+  software_statement: z.string().optional(),
 }).strip();
 
 /**