add RPC-based AUTHN and AUTHZ support

[UiP9AV6Y]

this change adds both a new authenticator as well as an authorizer implementation using RPC communication.

the advantage over "External Authentication" is that those new plugins are only spawned once upon server start instead of with each auth request. this reduces resource problems in high request scenarios where the process IDs the system has available dwindle due to too many processes being created.

the advantage over "Plugin Authentication" is that it works on all platforms.

example plugins for each aspect (AUTHN/AUTHZ) have been implemented to demonstrate the feature as well as a simple test suite to verify the functionality.

relates to #337

[UiP9AV6Y]

github.com/hashicorp/go-plugin does support customizing the logging stack, github.com/cesanta/glog however, does not. as a result the plugin system logs on its own accord in a different format. the only way to remedy this, would be to refactor the app to use a more customizable logging solution (e.g. the stdlib log/slog)

[UiP9AV6Y]

the minimum golang version has been raised in order to conform to the requirements of the newly added dependencies.

some code changes are not stricly required for this feature but are a result of running gofmt against the codebase. the MR could be reduced by formatting the code in a separate MR so i could rebase the branch against it.

[UiP9AV6Y]

github.com/cesanta/docker_auth/auth_server/plugin has been implemented as separate module for plugin authors to import without having to pull in the entire dependency tree of the "main" application.