Provide cache directory as a named flag (and not as a loose argument) to compiler plugins

[LukaszKontowski]

Switch
. - directory where cache file will be saved, required (description from one of plugins)
to a named flag.

Both for CodecRegistrationCheckerCompilerPlugin and for DumpPersistenceSchemaCompilerPlugin.

• Update code (remember to add descriptions in override val optionsHelp)
• Update default configs in the sbtAkkaSerializationHelper sbt plugin (module)
• Update README.md and docs/GUIDE.md
• Check in known projects that use ASH - if this configuration has been used somewhere. If yes, inform the right people about this change in upcoming release.

[PawelLipski]

If yes, inform the right people about this change in upcoming release.

Or even better, leave the support for the loose argument but mark as deprecated (emit a non-fatal warning from compiler plugin when used... TBD what if folks have -Xfatal-warnings enabled though)