PublicSettings exposes Option[PublicKey] - but it should be essential?

[rtyley]

PublicSettings exposes Option[PublicKey]:

pan-domain-authentication/pan-domain-auth-verification/src/main/scala/com/gu/pandomainauth/PublicSettings.scala

Line 48 in 59c7132

def publicKey: Option[PublicKey] = agent.get()

This is unlike PanDomainAuthSettingsRefresher which makes its PanDomainAuthSettings value non-optional:

pan-domain-authentication/pan-domain-auth-core/src/main/scala/com/gu/pandomainauth/PanDomainAuthSettingsRefresher.scala

Line 40 in 59c7132

def settings: PanDomainAuthSettings = authSettings.get()

...making the value optional seems weird - better to simply fail immediately if the system is unable to load credentials?

Talked this through with @andrew-nowak, we agreed it would be best to standardise on making the value non-optional.