Return 429 and 503 occasionally

[jcjones]

The Let's Encrypt API (overall, through a combination of Boulder and the load balancers) returns code 503 when the servers are overloaded, and code 429 when a given client is retrying too often, both with a Retry-After header.

Pebble should return those codes occasionally, too.

[webprofusion-chrisc]

Adding to this we've recently seen a couple of CAs report more rate limiting and server busy type messages which not all clients are handling all that well. It would be great if there was a Chaos mode which could (with configurable frequency/bias) return:

• 429 rate limits, optionally with Retry After
• 503
• 500
• 200 OK, with empty response when a response was expected
• very slow finalization or very slow auth (e.g. several minutes)
• perhaps make it configurable which endpoints would participate in the above.