Functions to resolve and call a Summon provider.

func Resolve(providerArg string) (string, error)

Searches for a provider in this order:

1. providerArg, passed in via CLI
2. Environment variable SUMMON_PROVIDER
3. Executable in /usr/libexec/summon/.

func Call(provider, specPath string) (string, error)

Given a provider and secret's namespace, runs the provider to resolve the secret's value.