Add coverage for the quarkus-spring-boot-properties Quarkus extension

[Sgitario]

Exploratory testing for the quarkus-spring-boot-properties Quarkus extension. The application consists of a REST endpoint
with some different approaches to inject properties.

Current limitations:

• Relaxing name convention is not supported and it won't be supported: Spring @ConfigProperties bindings are not relaxed quarkusio/quarkus#12483
• The annotation @ConstructorBinding is not supported yet: Spring Properties: Support of @ConstructorBinding when using @ConfigurationProperties quarkusio/quarkus#19364

Fix #172

[jsmrcka]

The code looks fine.

What's the situation on quarkusio/quarkus#19364? The feature might become supported in the future, yet there is no mention of it in the code.
On the other hand, there are 3 other features (quarkusio/quarkus#19365, quarkusio/quarkus#19366, quarkusio/quarkus#19368), which are almost certainly not going to be supported, but they are mentioned in the code and a coverage for them is present in the form of commented-out code.

[Sgitario]

The code looks fine.

What's the situation on quarkusio/quarkus#19364? The feature might become supported in the future, yet there is no mention of it in the code.
On the other hand, there are 3 other features (quarkusio/quarkus#19365, quarkusio/quarkus#19366, quarkusio/quarkus#19368), which are almost certainly not going to be supported, but they are mentioned in the code and a coverage for them is present in the form of commented-out code.

As I created the issues myself, I will cover those scenarios for sure. About the three other issues... I don't think it's 100% sure that it won't be supported (not in the short term for sure, but maybe in the long).