Update faas-cli new to use stack.yaml as a default

[alexellis]

Description

Update faas-cli new to use stack.yaml as a default

Updates faas-cli new to create a stack.yaml file instead of a YAML file named after the function. This is to help users discover that stack.yaml does not require a --yaml or -f flag which can become cumbersome.

When a stack.yaml or the file specified via -f already exists the new command will now prompt users to use --append

Additional changes:

• The signing option for faas-cli invoke changes to the more modern SHA256, I don't believe this is actually used by anyone so didn't retain the SHA1 option
• go.mod and vendor updated for third-party dependencies

Motivation and Context

Requested a number of times over the years, I also find it more convenient to work from a file where a separate -f parameter isn't needed for each command

How Has This Been Tested?

Various manual tests and the unit tests needed updates since there were test tables which wrote out different names which started to conflict with the new behaviour described.

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:

Certain blog posts may need an update if there are any commands such as mv fn.yml stack.yml, which could be tricky to find. Perhaps a regex in the docs and the website would be a good starting point.