0.5.0

New optional products for Vercel features, each its own library so apps only link what they use.
See "Optional Vercel features" in the README.

• VercelCache: the Vercel Runtime Cache (RuntimeCache), like getCache() from
  @vercel/functions and sharing entries with it: Codable values with TTLs and tags (dates
  encoded like Node's JSON.stringify, customizable with encoder:/decoder:), and an in-memory
  fallback locally.
• VercelGlobalConfig: reads Global Config (formerly Edge Config) stores (GlobalConfig), like
  @vercel/edge-config: get, has, getAll and digest, typed or as GlobalConfigValue.
• VercelFirewall: rate limiting with Vercel Firewall rules (Firewall.checkRateLimit, returning
  a RateLimitResult with an optional RateLimitResult.Reason), like @vercel/firewall.
• VercelOIDC: the request's OIDC token and its claims (OIDC.token(), VercelOIDCToken), token
  exchange for other audiences (OIDC.exchange), and AWS credentials from STS
  (AWSCredentialsProvider, OIDC.awsCredentials).
• Each feature module throws its own error type (RuntimeCacheError, GlobalConfigError,
  VercelFirewallError, VercelOIDCError) with timedOut, requestFailed and an unexpected
  status case. Descriptions, dump and debug output of credentials, tokens and clients never
  show secrets.
• Vercel: everything above plus VercelRuntime, with one import Vercel.
• VercelRuntime: Vercel.withRequest now also works locally and sets Vercel.currentRequest,
  which the feature modules use to find the request's headers. Vercel.deadline and
  request.headers.vercel.deadline give the request's deadline. Vercel.reportFetch shows
  outbound requests under External APIs in Observability; the feature modules report theirs
  automatically.
• VercelBootstrap: passes the request deadline to the app as x-vercel-deadline (replacing any
  sent by a client), forwards fetch metrics, and hands runtime cache configuration that Vercel
  hides from request headers to the app over its socket (asked once per request, with a 1 second
  timeout). It no longer depends on VercelRuntime, so it stays small.