Enable injecting sidecar container via helm configuration.

[NicoletaPopoviciu]

Changes proposed in this PR:

Enable injecting sidecar container via helm configuration.
Based off Vault implementation https://github.com/hashicorp/vault-helm/pull/87/files#

• Add server.extraContainers and client.extraContainers to values.yaml
• Change client-daemonset.yaml and server-statefulset.yaml to check for extraContainers
• Add bats tests

How I've tested this PR:

• ran bats tests locally
• tested locally that the additional containers were created when extraContainers was specified
• helm install -f config.yaml nicoleta charts/consul

Sample config.yaml file used:

server:
  replicas: 1
  extraContainers:
    - name: static-server
      image: docker.mirror.hashicorp.services/hashicorp/http-echo:latest
      args:
        - -text="hello world"
        - -listen=:8080
      ports:
        - containerPort: 8080
          name: http
client:
  extraContainers:
    - name: static-server
      image: docker.mirror.hashicorp.services/hashicorp/http-echo:latest
      args:
        - -text="hello world"
        - -listen=:8080
      ports:
        - containerPort: 8080
          name: http

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...)

[ishustava]

Looks great to me 🎉 I've added a couple of minor edits, but nothing blocking

[ishustava]

Since this is an array of objects/maps

Suggested change

	# @type: array<string>
	# @type: array<map>

[lkysow]

I think the @type line needs to be the last in the comment. For example:

  # Example:
  #
  # ```yaml
  # imagePullSecrets:
  #   - name: pull-secret-name
  #   - name: pull-secret-name-2
  # ```
  # @type: array<map>

[ishustava]

Suggested change

	# @type: array<string>
	# @type: array<map>

[lkysow]

Looks great! Just some small nits.

[lkysow]

I think the @type line needs to be the last in the comment. For example:

  # Example:
  #
  # ```yaml
  # imagePullSecrets:
  #   - name: pull-secret-name
  #   - name: pull-secret-name-2
  # ```
  # @type: array<map>

[lkysow]

Suggested change

	@test "client/DaemonSet: no extra client containers added" {
	@test "client/DaemonSet: no extra containers added by default" {