Add config parser

[mostafa]

I used koanf to introduce a config parser that loads and parses a gatewayd.yaml file and runs the server. The config parser works for the current setup with a few missing config options, which will be added later. The config parameters will be updated as the project is developed.

This is the command line options for now:

$ ./gatewayd
GatewayD is a cloud-native database gateway and framework for building data-driven applications. It sits in between your database(s) and your database client(s) and proxies all queries to and their responses from the database.

Usage:
  gatewayd [command]

Available Commands:
  completion  Generate the autocompletion script for the specified shell
  help        Help about any command
  run         Run a gatewayd instance

Flags:
  -h, --help   help for gatewayd

Use "gatewayd [command] --help" for more information about a command.

And the run subcommand accept a single parameter (actually two including -h):

 ./gatewayd run --help
Run a gatewayd instance

Usage:
  gatewayd run [flags]

Flags:
  -c, --config string   config file (default is ./gatewayd.yaml) (default "./gatewayd.yaml")
  -h, --help            help for run