The plugin class loader from Pf4J looks for jars under target/lib
to use to load any classes that are missing target/classes. This
PR adds the `maven-dependency-plugin` to copy the jars to this
folder automatically so the classes are available during
development.
This is to simplify the query pane on the frontend client. The client doesn't need to have separate interfaces for sql and non-sql plugins. All queries will be sent to the server in the form of a String that is parsed in different formats based on the plugin.
Also adding test cases for PostgresPlugin. Used TestContainers to simulate the postgres db in Docker inside the Java test itself. Very useful.
We are moving to a multi-module structure so that different parts of the codebase can be exposed to the public while others can remain private. Using pf4j for plugin framework.
Also adding a build script `build.sh` which compiles the code and creates the `dist` folder for distribution purposes. Now we can build the code via
```
$ ./build.sh -DskipTests
```
