Add support for configurable envoy proxy concurrency

[kschoche]

Changes proposed in this PR:

• Adds a new flag to connect-inject deployment to take in a default concurrency value for every envoy sidecar.
• Adds support for a new annotation on connect injected pods consul.hashicorp.com/consul-envoy-proxy-concurrency which will override the default for any given pod.

Notes to reviewers:

• The default setting of 2 was chosen via a brief discussion with @blake and @david-yu but I'm open to suggestions if we have more information. Additional reading on this can be had at https://blog.envoyproxy.io/envoy-threading-model-a8d44b922310 where they encourage setting the value very low for sidecars and high for edge proxies.
• I'm also open to suggestions on naming conventions for the various flags, I realize putting the word envoy into the name is a break from our typical conventions, but also sidecar in this context is muddy (consul-sidecar vs envoy proxy sidecar) and I wanted it to be clear that it was an envoy specific flag.
• I chose not to just force the user to add --concurrency 42 to their connectInject.sidecarProxy.extraArgs because I thought it was a bit messy to have to edit strings in deployments vs just adding a new annotation to your deployment or set a default via helm.

How I've tested this PR:
unit tests + acceptance tests

How I expect reviewers to test this PR:
👀

Checklist:

• Tests added
• CHANGELOG entry added

  HashiCorp engineers only, community PRs should not add a changelog entry.
  Entries should use present tense (e.g. Add support for...)

[curtbushko]

Looks good! I only had some minor questions/nits but I think it all comes down to naming.

I prefer sidecar instead of envoy for consistency reasons. I think it is concurrency that is the word that makes it confusing as I immediately thought of replicas when I read the summary.

Would thread-concurrency make it clearer? -default-sidecar-proxy-thread-concurrency?

Naming is hard. 🥲

[thisisnotashwin]

NICE