Rename binary to `function`

[lance]

Based on conversation during test day, the consensus is that the binary and subsequent kn plugin should be renamed to functions. This has implications both in the name of the binary, but also in the CLI help strings as the faas.yaml filename.

[lkingland]

function or functions?

[zroubalik]

I suppose it should be function

[zroubalik]

We should probaly rename env variables, such as $FAAS_CONFIRM,$FAAS_IMAGE, $FAAS_PATH and$FAAS_REGISTRY.

And what about quay.io/boson/faas ?

[zroubalik]

Then we should follow up with renaming this repo from boson-project/faas -> /boson-project/function and change the occurrences in code (package..)

[lance]

Also, this should include rename of faas.yaml to function.yaml.