Add Helm chart for Google Cloud Pub/Sub connector

[welteki]

Description

Add Helm chart for Google Cloud Pub/Sub connector.

Why is this needed?

• I have raised an issue to propose this change (required)
• [ ]

Allow users to deploy the OpenFaaS gcp-pubsub-connector with Helm.

Who is this for?

What company is this for? Are you listed in the ADOPTERS.md file?

How Has This Been Tested?

Deployed the connector in my local cluster using the chart.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[alexellis]

There are a couple of Makefile targets that will need an update please.