Always have the filesystem plugin available

[carolynvs]

Is your feature request related to a problem? Please describe.
I want to always be able to use credentials from env vars, but sometimes I want secrets too. I'm complicated like that.

Describe the solution you'd like
I want to be able to activate a secrets plugin but also have the filesystem plugin available if I am using env, path, value ,etc in my cred set.

Describe alternatives you've considered
NOTHING THIS IS THE ONLY WAY 😉

Additional context
This will be applicable for when we have parameter sets in the future. Vaughn says so.

[vdice]

I spent some time researching what this might look like. As a first exercise, I wanted to see what it would look like to add the host secret store (as defined in cnab-go) to the only plugin we currently have that offers secret source resolution (azure.keyvault, from the porter-azure-plugins repo). The result can be seen here and it works great for me with a test bundle (wherein both parameter and credential sets include a mix of secret sources as well as env/value/etc. sources).

This approach would mean that future secrets plugins would need to add the fallback host store for non-secret sources (assuming support is desired). What do we think?

[carolynvs]

I was expecting that porter wouldn't make it the responsibility of the plugins to handle host keys. 🤔 But this is a really tiny workaround. Let's go with this for now and if something else comes to us that allows us to have Porter take care on this responsibility in the future, we can switch the implementation.

Thanks! 💯

[carolynvs]

This works just fine in porter now