Config File Handling

[alex-kar]

Handle config file .iceberg-go.yaml in the home directory.

Closes #62

@zeroshade I noticed that --catalog has different functionality compare to pyiceberg cli, where it defines catalog name to be loaded from configuration file.

iceberg-go - --catalog - catalog type.
pyiceberg --catalog - catalog name to load from configuration file.

Because of this reason, currently it can load only default catalog.

Another note is that docopt throw's exception when struct has some non default values.
docopt/docopt.go#55
Because of this limitation, I can't first parse configuration file and then provide the same struct to docopt to ensure the correct precedence. instead I added this manual mapping:

func mergeConf(fileConf *config.CatalogConfig, resConfig *Config) {
	if len(resConfig.Catalog) == 0 {
		resConfig.Catalog = fileConf.Catalog
	}
	...
}

[zeroshade]

should we probably add an option to specify the config file if they want something other than the default .iceberg-go.yaml?

[alex-kar]

@zeroshade I have added --config option to specify the path to the configuration file.

[zeroshade]

LGTM thanks!!

[nastra]

@alex-kar can you please rebase and fix the merge conflict?

[alex-kar]

@nastra Rebased onto the latest main.