PCCS - allow configuring an API key for Intel PCS

[ameba23]

Currently the PCCS implementation uses Intel PCS anonymously. This means we are rate limited together with all other anonymous users.

For pre-warm, where we fetch all available collateral and make many API requests, this is an issue.

We could add an option to add an Intel API key.

[samlaf]

The discussion in #88 (comment) about the best API for the Pccs struct led me here.

Think addressing this issue could even make the whole constructor API surface cleaner actually! I'd propose replacing the pccs_url with:

  pub enum CollateralSource {
      /// Intel PCS. A subscription key lifts the anonymous rate limit.
      IntelPcs { subscription_key: Option<String> },
      /// Any PCCS-compatible service: self-hosted, Phala, a local sidecar.
      Pccs { url: String },
  }