Add fetch API handlers and clients

[timostamm]

This adds the exports createFetchClient and createFetchHandler from @bufbuild/connect/protocol:

/**
 * Create a universal client function, a minimal abstraction of an HTTP client,
 * using the given fetch() implementation.
 */
function createFetchClient(fetchFn: typeof fetch): UniversalClientFn;

/**
 * Convert a universal handler function to a fetch handler.
 */
function createFetchHandler(uHandler: UniversalHandlerFn, options?: FetchHandlerOptions): FetchHandlerFn;

The two functions can be used with the transports and handlers from @bufbuild/connect/protocol-*. This is the first step towards supporting edge runtimes.