Add config validation

[dk1844]

Background

Add config validation logic to ensure that the configuration values provided are sensible and LGW can work with these.

Feature

This may be something completely custom, but it would be nice if we could use some already established validation options, too.

Solution Ideas:

1. Write custom checks on the application start
2. Consider leveraging existing JSR-303 validation approach with Springboot's @Valid and @Validation annotations. Related article. Just beware, that we use @ConstructorBinding that may not work with @ConfigurationProperties
3. Any other solution or combination of the above.