Add sonar coverage reporting to master build

[jimschubert]

Submits analysis to sonar on master branch build.

New sonar project is at https://sonarcloud.io/dashboard?id=OpenAPITools_openapi-diff

This submits sonar for the Java 11 build. Sonar will complain for any code submitted and built against Java 8, as support for Java 8 will be removed soon. This also adds and configures Jacoco for coverage instrumentation in the root pom. The sonar.branch.name configuration is dynamic as this would allow extending this workflow for other branches without submitting these all to master.

As a side note, there's no clean way to run a sonar PR check because Sonar requires a secret token and these are not available in the pull_request event. We might be able to work something out with the pull_request_target event (which runs in the scope of the master branch), but that would require a bit of work and poses a potential security risk. Sonar also doesn't support automated checks for compiled languages.

I haven't included a sonar badge in the readme here. Some options are below.

"official" badge

[![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=OpenAPITools_openapi-diff&metric=alert_status)](https://sonarcloud.io/dashboard?id=OpenAPITools_openapi-diff)

Renders as:

Coverage Shield

![Sonar Coverage](https://img.shields.io/sonar/coverage/OpenAPITools_openapi-diff?color=blue&label=Code%20Coverage&server=https%3A%2F%2Fsonarcloud.io&style=flat-square)

Renders as:

Quality Gate Shield

![Sonar Quality Gate](https://img.shields.io/sonar/quality_gate/OpenAPITools_openapi-diff?label=Quality%20Gate&server=https%3A%2F%2Fsonarcloud.io&style=flat-square)

Renders as:

Total Tests Shield

![Sonar Test Count](https://img.shields.io/sonar/total_tests/OpenAPITools_openapi-diff?label=Total%20Tests&server=https%3A%2F%2Fsonarcloud.io&style=flat-square)

Renders as:

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
9 Code Smells

0.0% Coverage
1.5% Duplication