support for proxying multiple BEE_API_URLs

[ferencsarai]

Copied from: ethersphere#501

• allow comma separated values for BEE_API_URL
• cycle requests between those URLs as a simple load balancer

1. config.ts
const beeApiUrl = BEE_API_URL || DEFAULT_BEE_API_URL split by comma and pluralise

2. proxy.ts

   1. Change interface Options, replace beeApiUrl: string with string[]
   2. Pick one in beeApiUrl in fetchAndRespond

3. stamps.ts
   Rewrite get postageStamp(): string with custom logic to select stamp (or ignore if using custom middleware for stamps)

4. server.ts
   There is a Bee instance created in server.ts:

   const bee = new Bee(beeApiUrl)

   However, it is used only for readiness checks, exposing Bee identity, and homepage. If you don't need those features, feel free to ignore it.

   Simple but ugly fix is using beeApiUrls[0] instead.