Relax the file-existence rigor for manifest.json files

[colearendt]

Without a proper .connectignore, etc. it is tricky / painful to have github actions fail when a file does not exist (i.e. was not committed to github) that is being shown in the manifest.json

We should relax the error if files do not exist when we are checking the manifest.json "files" arg.

[aodhan-domhnaill]

Is it possible to have the manifest.json to be automatically built by an action previous to where it's needed?

I see this for Python https://github.com/rstudio/rsconnect-python#creating-a-manifest-for-future-deployment, but I can't find one for R.