Use POJO models to load configurations with snakeyaml

This issue is for loading configurations to POJO models with `snakeyaml`. Currently all loads default to `Map` and must have all components manually checked. This could be swapped with the following examples:

- https://codeberg.org/snakeyaml/snakeyaml/wiki/Documentation#providing-the-top-level-type
- https://codeberg.org/snakeyaml/snakeyaml/src/branch/main/src/test/java/org/yaml/snakeyaml/Example2_27Test.java
- https://codeberg.org/snakeyaml/snakeyaml/src/branch/main/src/test/java/org/yaml/snakeyaml/Invoice.java

Which show how to use the constructor of a class as the basis for loading a YAML file. This should be applied everywhere a `yaml.load` occurs:

- https://github.com/canonical/devpack-for-spring-cli/blob/c26d227cffc3f6a688b01e6b0f08252d5e228e26/src/main/java/com/canonical/devpackspring/build/PluginDescriptorContainer.java#L39
- https://github.com/canonical/devpack-for-spring-cli/blob/c26d227cffc3f6a688b01e6b0f08252d5e228e26/src/main/java/com/canonical/devpackspring/setup/SetupModel.java#L34
- https://github.com/canonical/devpack-for-spring-cli/blob/c26d227cffc3f6a688b01e6b0f08252d5e228e26/src/main/java/com/canonical/devpackspring/snap/Manifest.java#L42
- https://github.com/canonical/devpack-for-spring-cli/blob/c26d227cffc3f6a688b01e6b0f08252d5e228e26/src/main/java/org/springframework/cli/command/SetupCommands.java#L72
- https://github.com/canonical/devpack-for-spring-cli/blob/c26d227cffc3f6a688b01e6b0f08252d5e228e26/src/main/java/org/springframework/cli/command/SetupCommands.java#L78

Start with the yaml.load calls in PluginDescriptorContainer.java, SetupModel.java, Manifest.java, and both locations in SetupCommands.java. Read the SnakeYAML top-level type example and Example2_27Test.java, then compare the referenced model classes. Done means each listed load uses the appropriate POJO type instead of defaulting to Map.

Written by the indexing model from the issue text.