Parse addons template outputs to detect secrets

[efekarakus]

Task

We want to parse the Outputs section of the addons template generated in #646 to detect if an output value refers to a secret.

Programming model

See this gist for an example on how to do this with awslabs/goformation: https://gist.github.com/efekarakus/14814a0043905e5242626922b01ea859

package addons

type Outputs structs {
     ManagedPolicyARN string
     EnvVars []string
     Secrets []string
}

func (a *Addons) Outputs(template string) (*Outputs, error)

The Outputs object can then be passed into the web app template to inject secrets, env variables, and the managed policy!