feat: upgrade to go 1.18 and support modules via workspaces

[LucasRoesler]

Description

Support Go 1.18 and improve module support using Workspaces

Motivation and Context

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

Which issue(s) this PR fixes

Fixes #

How Has This Been Tested?

See https://github.com/LucasRoesler/openfaas-golang-template-workspace

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [?] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Version change (see: Impact to existing users)

Impact to existing users

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] 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]

Examples that come to my mind:

• Private Go module usage, perhaps with vendoring
• Named sub-packages i.e. routes, imported into the handler
• Normal Go modules with external packages
• Normal Go modules on, with public vendored packages?

[LucasRoesler]

@alexellis i remove the go env statement and i have updated the readme here https://github.com/LucasRoesler/openfaas-golang-template-workspace which explains the supported modes, please make sure to review that before we merge.

[alexellis]

I took a look at the examples repo, the only thing that would make them better is dropping a CI build into the repo to validate them automatically.

I have an example you could adapt in this blog post