ruby: enable coverage report

[AdamKorcz]

This enables coverage reports for Ruby projects.

The following was required:

1. Both base-builder-ruby and base-runner need the SimpleCov dependency which is a library for generating coverage reports for Ruby projects.
2. SimpleCov creates a coverage report that consists of several files. I have added scripts that merge that coverage report into a single html file. These scripts are infra/base-images/base-runner/consolidate_ruby_coverage.sh and infra/base-images/base-runner/consolidate_html.py.
3. I have added a higher-level Ruby module that users should invoke instead of Ruzzy.fuzz(). There are two reasons for this: 1) I could't get coverage to work by calling Ruzzy.fuzz() - it doesn't work well with SimpleCov, and 2) Ruzzy.fuzz() was preventing SimpleCov from working properly. The difference is very small: Instead of users calling Ruzzy.fuzz(), they now do require 'ossfuzz_helper' and then run OSSFuzz.fuzz(). The ox-ruby fuzzers demonstrate this.

To test this:

cd /tmp
git clone https://github.com/AdamKorcz/oss-fuzz --branch=ruby-coverage-report
cd ruby-coverage-report
# rebuild base images (./infra/base-images/all.sh)
mkdir -p build/corpus/ox-ruby/fuzz_parse
python3 ./infra/helper.py build_fuzzers ox-ruby
python3 ./infra/helper.py run_fuzzer ox-ruby fuzz_parse --corpus-dir=$PWD/build/corpus/ox-ruby/fuzz_parse
# kill it after a few seconds
python3 ./infra/helper.py build_fuzzers ox-ruby --sanitizer=coverage
python3 ./infra/helper.py coverage ox-ruby --no-corpus-download

[AdamKorcz]

/gcbrun trial_build.py all

[AdamKorcz]

None of the failures in the trial_build are caused by this PR:

Project	Failed builds	Already fails
typst	coverage	✅
uint256	coverage	✅
unirest-java	coverage	✅
unrar	coverage	✅
wamr	coverage	✅
yajl-ruby	coverage	✅
ygot	coverage	✅
upx	fuzzing	✅
urllib3	fuzzing	✅
wheel	fuzzing	✅
yoga	fuzzing	✅
vitess	fuzzing + coverage	✅
wasmer	fuzzing + coverage	✅
xmlbeans	fuzzing + coverage	✅
xmlsec	fuzzing + coverage	✅
xnnpack	fuzzing + coverage	✅
xpdf	fuzzing + coverage	✅
xstream	fuzzing + coverage	✅
unicorn	introspector	✅
uwebsockets	introspector	✅
wabt	introspector	✅
wazuh	introspector	✅
wireshark	introspector	❌ (times out)
wget2	introspector	✅
wxwidgets	introspector	✅
xerces-c	introspector	✅
wolfssl	introspector	✅
xnu	introspector	✅
wget	fuzzing + coverage + introspector	✅
wolfmqtt	fuzzing + coverage + introspector	✅
xvid	fuzzing + coverage + introspector	✅
zeek	fuzzing + coverage + introspector	✅