refactor: use functional options pattern for config

[dirodriguezm]

Implementation of the functional options pattern.

It allows to configure the server like this

var config tapsync.Config
config = tapsync.NewConfig() // by default config uses env variable for DB url
// but can also be configured like this
config = tapsync.NewConfig(WithDatabaseURL("url"), WithPort(5000))
service := tapsync.NewTapSyncService(config)