Solves a single thing in the build configurations, resulting in a few
wins.
1. Reduced number of warnings in the output.
1. In release branch:
```
mvn clean package -DskipTests | grep --fixed-strings --count '[WARNING]'
3233
```
1. In this PR's branch:
```
mvn clean package -DskipTests | grep --fixed-strings --count '[WARNING]'
172
```
2. All uber-jar files are shaded twice, currently. Once with the default
execution of `maven-shade-plugin`, and again with the `shade-plugin-jar`
execution in these `pom.xml` files. This is double-work, and is the
cause of most of the warnings we see.
1. This `shade-plugin-jar` was added to have the plugin information
included in the `/META-INF/MANIFEST.MF` file, since we can't configure
the default execution of the shade plugin (it comes to us from Spring
Boot).
2. Instead, we switch to configuring plugin information in a
`/plugin.properties` file.
3. Previously, we used `/plugin.properities` for plugin information in
dev time, and `/META-INF/MANIFEST.MF` in production. This PR will change
it so that we use `/plugin.properties` all the time. We configure PF4J
with a custom plugin manager to achieve this.
3. Moved all `plugin.properties` into `src/main/resources`, so that they
land up in the root of the final jar files. But this means, during
development, loading the plugin fails since it looks for a
`plugin.properties` at the root of the plugin module, i.e., next to the
`src` folder.
1. For this, in the custom plugin manager class, we change where we look
for the `plugin.properties` file during development mode. In this mode,
we look at the `target/classes/plugin.properties` file, which is where
maven saves this file, taken from
`src/main/resources/plugin.properties`.
2. This also solves the duplication of the plugin properties that's
currently present, between `plugin.properties` and the `<properties>`
section of `pom.xml` files.
Here's the shade plugin's default execution and configuration, from
Spring Boot:
https://github.com/spring-projects/spring-boot/blob/v3.0.1/spring-boot-project/spring-boot-starters/spring-boot-starter-parent/build.gradle#L174.
4280ded88e