Missing features in the LCPService API

[mickael-menu-mantano]

A few features are missing currently in the LCPService:

• Retrieving the LCPLicense object from a LCPL or JSON.
• Importing a publication from a license's JSON directly, instead of a LCPL file on the file storage.
• Pre-load one or more user passphrases in the LCP database. (Various API improvements r2-lcp-swift#97)
• Having access to the LCPLicense after calling importPublication (in the LCPImportedPublication)

To avoid multiplying the functions, we can add something like this:

extension LCPLicenseSource {
    case lcpl(URL)
    case publication(URL)
    case json(String)
}

func importPublication(from source: LCPLicenseSource, ...)
func retrieveLicense(from source: LCPLicenseSource, ...)