Support to load a single value

[jansupol]

Currently, Jakarta Config allows for loading config into an interface

@Configuration
public interface A {
   String value();
}

The most use-cases, however, are as follows:

if (config.property('NAME1')) {
     // do something
}
if (config.property('NAME2')) {
   // do something else
}

The convenient way is to allow the possibility to load the single value as Loader.bootstrap().path("NAME1").asString().
In MP config, there is a similar notion: config.getValue("NAME1", String.class)

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There are the following possibilities:

1. Allow just few methods: asInt(), asBoolean, asString
2. Allow all primitive types: boolean, byte, short, float, double, int, long, char, and String
3. Allow just asString() and more generic as(Class<T> type) and as(TypeToken type) - this has an advantage that later on the config could support as(UserDefinedType), but a disadvantage that we should define a way to pass convertors to the Loader.

The use-case usually checks whether the property is present. That can be implemented by:

1. returning null when not available
2. returning Optional<> (my favourite)
3. throwing NoSuchObjectException
4. implement a complementary isProperty()

In the case of Optional, it would be possible to use:
loader.path("my.property").asString().ifPresent(myProperty -> doSomething(myProperty))

[jansupol]

Out of the discussion, should we keep load(Interface.class), or would it better be as(Interface.class) which then can be as(String.class) as well?