[RFC] Storage of code coverage results in KCIDB

With https://github.com/kernelci/kernelci-pipeline/pull/1211 Maestro will gain the ability to extract the code coverage results (functions and lines coverage percents) for each test *and* consolidate/merge those results at the build level.

In order to make this data more useful to end users, we need to store it in KCIDB so we can display historical data and trends in the dashboard. This would e.g. provide factual data points supporting the addition of new tests or spotting regressions due to failed or modified tests.

The plan so far is to store the coverage results in a new table, referencing the build on one hand, and storing the following data as separate fields:
* percentage of lines covered
* percentage of functions covered
* URL of the generated HTML report (which includes coverage data for each source file)

Individual test results doesn't seem relevant as those aren't supposed to change over time.

Comments and suggestions are obviously welcome.

cc @padovan

The issue names no files, tests, or entry points; begin by reviewing KCIDB's existing build-related schema and storage paths. Define the scope for a new table linked to builds with line coverage, function coverage, and an HTML report URL, then confirm how the data will support historical dashboard trends.

Written by the indexing model from the issue text.