Set User-Agent header on Consul API calls

[lkysow]

• Upgrade to latest consul api package
• Create common consul package that implements NewClient(). This
  function returns a Consul client that will set the header
  User-Agent: consul-k8s/<version> on all calls to Consul.
• Swap to using this new constructor everywhere.
• Add a linter that ensures we use our NewClient() function everywhere
  in the future. Cribbed from https://github.com/hashicorp/lint-consul-retry.

How I've tested this PR:

• unit test

• ran a request printer: docker run -p 8501:8443 --rm -t mendhak/http-https-echo:17

  then ran CONSUL_HTTP_SSL_VERIFY=false consul-k8s get-consul-client-ca -output-file /tmp/out -server-addr localhost -server-port 8501

  "GET /v1/agent/connect/ca/roots HTTP/1.1" 200 458 "-" "consul-k8s/0.23.0-dev"
  -----------------
  {
      "path": "/v1/agent/connect/ca/roots",
      "headers": {
          "host": "localhost:8501",
          "user-agent": "consul-k8s/0.23.0-dev",
          "accept-encoding": "gzip"
      },
      "method": "GET",
      "body": "",
      "fresh": false,
      "hostname": "localhost",
  

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)

[kschoche]

This is great stuff, and I like the linter too!
Tested it out and looks good.
I left one comment which isn't a blocker but I'd like to get other opinions on whether or not we should keep using DefaultConfig() from API or duplicate it inside the new lib from a code readability standpoint.
So cool

consul-k8s % go run hack/lint-api-new-client/main.go
Found code using github.com/hashicorp/consul/api.NewClient()
instead of github.com/hashicorp/consul-k8s/consul.NewClient()
in the following files:
-  github.com/hashicorp/consul-k8s/connect-inject/health_check_resource.go
exit status 1

[ndhanushkodi]

Looks great!!