Pluggable storage

In common CI/CD use-cases a lot of the space is being consumed by test reports. This data is stored within JENKINS_HOME, and the current storage format requires huge overheads when retrieving statistics and, especially, trends. In order to display trends, each report has to be loaded and then processed in-memory.

The main purpose of externalising Test Results is to optimize Jenkins logic by querying the desired data from specialized external storages, e.g. from Document-based databases like Elasticsearch. According to the current plan, JUnit Plugin will be extended in order to support such external storage in its APIs being widely used by test reporting plugins.

## Status:

Foundation work started

Prototype API: https://github.com/jenkinsci/junit-plugin/pull/110

Continued in: https://github.com/jenkinsci/junit-plugin/pull/141

## Todo:

- [x] New trend chart implementation that doesn't rely on loading Runs
- [x] Rewrite history chart, probably port it to echarts
- [x] Fix package result loading showing no results when viewing by package
- [x] Plugin implementing API - most likely a PostgreSQL plugin
- [x] Benchmarks - show performance improvements for large instances
- [ ] Present at Cloud native Sig
- [ ] Announcement blog on Jenkins.io

## Test strategy:

The junit plugin will provide a sample implementation using h2 database that can be used to verify pluggable storage functionality.

The Postgres implementation will be able to use something like test containers and replicate a similar test inside it's own plugin.

Performance tests will be written using the Microbenchmark harness, see https://www.jenkins.io/blog/2019/06/21/performance-testing-jenkins/

## Graduation criteria

This API will be released in Beta initially and will likely change as it's developed.

* At least one production ready pluggable storage implementation
* Benchmarks showing improvements
* Blog on Jenkins.io