Move parser class to parser package

[lavenderses]

What

package re-organizing related to parser class

Why

Before refactoring, parser is in app_config_model package, which is the DTO for response from AWS AppConfig.

I think that much more parse function will be added with #5 to AppConfigParser.
So, to clearfy package structure, re-organize it first.

In the next PR, AppConfigParser#attributeAsObject and AppConfigParser#attributeAsBoolean will be moved to outer class.

How

Move AppConfigParser, AppConfigParseException and its test to parser package.

Notes

N/A

[lavenderses]

LGTM