add Secret Store implementation

[joeshaw]

This adds hostcall and SDK support for the Fastly Secret Store. An
example is included for usage but the basic idea is:

st, err := secretstore.Open("example_secretstore")
if err != nil {
    panic(err)
}

s, err := st.Get("my_secret")
if err != nil {
    panic(err)
}

pt, err := s.Plaintext()
if err != nil {
    panic(err)
}

fmt.Fprintf(w, "secret value: %q", v)

[joeshaw]

Still to do: Viceroy doesn't have support for these hostcalls yet, so the example is expected to fail on it. Looks like we need an integration test for it as well (which I would expect to fail for now).

[joeshaw]

Viceroy PR: fastly/Viceroy#210

[dgryski]

LGTM