Use HTTP 414 status code for GET maxItems enforcement

[jshufro]

I'd like to make the case for why these codes should be implemented-

1. The 400 description doesn't mention maxItems: Invalid state or validator ID, so a BN enforcing maxItems will potentially confuse a downstream system.
2. The BN isn't always directly responsible for responding to the downstream system- for example, nginx, cloudflare, haproxy, etc may return a 414 if the Request-Line is too long. Enshrining the 414 code in the spec serves as a signal to programmers of downstream systems that they ought to take care to handle those errors.
3. A client team developing both a VC and a BN may incorrectly assume that because their BN never returns a 414, they do not need to handle a 414 in the VC.

[rolfyone]

LGTM