Merge of features and code refactoring developed as part of OBDF experiments

[fracorco]

This PR covers additional features and substantial code cleanup/refactoring carried out for the OBDF experiments.

New/improved features:

• added QuerySelector abstraction to instantiate query templates, keeping previous SQL-based implementation and adding new CSV-based one
• added plugin mechanism (for Mixer and QuerySelector interfaces) with runtime retrieval of available plugins and their options/metadata
• extended logging: import of statistics from external files, input/output hashes in generated mixer.json statistics; refactoring of statistics code
• building of executable fat jar via shade + really-executable-jar-maven-plugin (vs assembly plugin), with updated build.sh script
• support for conditionally retrying queries (based on regex matching of error message, up to a number of times and with wait time between retries)
• support for waiting a configurable time between successive queries
• support for ending warm up based on a configurable timeout
• added Dockerfile

Code cleanup/refactoring:

• consolidation of code into fewer modules; removed mixer-distribution, mixer-utils, mixer-db-connection
• consolidation of .gitignore files into a single repository-level one
• optimization of Maven configuration, with parent-level dependencies/plugins/profiles management
• reworked/improved timeout handling
• refactoring and simplification of code for parsing options from command line arguments
• specification of argument/return value nullability via JSpecify annotations
• removed out-of-scope files from the repository (Eclipse and IntelliJ project files, OBDF configurations)
• ensuring that response stream from SPARQL endpoint is fully consumed and closed, even in case of a timeout
• documentation (readme, javadoc for core classes)

Updated project requirements:

• Java 11+ (on the basis that Ontop also requires Java 11+).
• Maven 3.6.3+

Discussion items:

• whether (and where) to publishing the Docker image
• whether to maintain support for embedding JDBC drivers
• whether to increase obda-mixer version (proposal: 1.3)