WFE: Add new key-value ratelimits implementation

[beautifulentropy]

Integrate the key-value rate limits from #6947 into the WFE. Rate limits are backed by the Redis source added in #7016, and use the SRV record shard discovery added in #7042.

Part of #5545

---

Notes for reviewers:

There were a lot of small changes necessary to complete this change:

Changes in ratelimits:

• Make Name an fmt.Stringer and utilize the new String() method throughout.
• Improve documentation for NewRegistrationsPerIPv6Range.
• Add metrics similar to those added to our existing rate limits in ratelimit: Overhaul metrics for the our existing rate limits #7054
• Changes to Redis Source:
  • Register go-redis client metrics at *redis.Ring construction, outside of the Redis Source. These metrics are germane to client itself, not the source implementation in ratelimits.
  • Remove ratelimits.RedisSource.timeout. We should instead rely on the go-redis read and write timeouts.

Changes in bredis:

• Copy rocsp_config.RedisConfig to bredis.Config.
• Add utility function MustRegisterClientMetricsCollector() for registering go-redis client metrics.
• Add SRV lookup fields to the new bredis.Config and a new constructor NewRingFromConfig() to handle go-redis client and service discovery construction. This is much cleaner than constructing a client inside of the WFE and will be useful if/when we add more Redis caches to the WFE.
• lookup.updateNow() now re-registers client metrics when new shards are discovered.

Changes in rocsp/config:

• Move redis client metrics registration from NewReadingClient() to MakeReadClient()
• MakeReadClient() calls the new bredis.MustRegisterClientMetricsCollector() for consistency.

Change in wfe2:

• Spend against the NewRegistrationsPerIP/v6Range inside the new checkNewAccountLimits() func and if the subsequent ra.NewRegistration() call fails, refund the spent limit quota using the new refundNewAccountLimits().

Changes in cmd/boulder-wfe:

• Construct the *redis.Ring client, the bredis.Lookup (optionally), and add the resulting Limiter to the resulting WebFrontEndImpl.

[github-actions]

@beautifulentropy, this PR appears to contain configuration changes. Please ensure that a corresponding deployment ticket has been filed with the new configuration values.

[pgporada]

//rocsp/rocsp.go NewReadingClient states rdb must be non-nil, but shouldn't there be a nil check inside the constructor for that rather than just a comment?

[beautifulentropy]

//rocsp/rocsp.go NewReadingClient states rdb must be non-nil, but shouldn't there be a nil check inside the constructor for that rather than just a comment?

I didn't actually write that comment. It shows green because I removed the content in that comment which came directly after it and was no longer applicable due to a change I made inside the function.

[aarongable]

Mkay, I've

[aarongable]

LGTM with the exception of just one question about the new context cancellation mechanism.