Allow bitcoin-s.datadir configuration key to be used instead of passing in a file

[Christewart]

This is how AppConfigFactory is defined currently.

This is weird, because we allow users (specifically when using bitcoin-s as a library!) to specify a configuration in their application.conf for bitcoin-s.datadir. You can see this on the website here: https://bitcoin-s.org/docs/next/config/configuration#example-configuration-file

If you look closely at AppConfigFactory, we have no option for constructing an AppConfig solely from the contents of the configuration file.

Right now I have to do something like this to construct a DLCOracleAppConfig() for instance

  val datadir: Path =
    Files.createTempDirectory(s"hello-world-${scala.util.Random.nextInt(100)}")
  val appConfig = DLCOracleAppConfig.fromDatadir(datadir)
  val dbConfig = appConfig.slickDbConfig

We need to create something like

AppConfig.fromConfig()