Allow test sharding for e.g. Bazel test sharding feature

This greatly simplifies running Catch2 tests in single binary
in parallel from external test runners. Instead of having to
shard the tests by tags/test names, an external test runner
can now just ask for test shard 2 (out of X), and execute that
in single process, without having to know what tests are actually
in the shard.

Note that sharding also applies to test listing, and happens after
tests were ordered according to the `--order` feature.

src/catch2/catch_config.cpp

@@ -73,6 +73,8 @@ namespace Catch {
     double Config::minDuration() const                 { return m_data.minDuration; }
     TestRunOrder Config::runOrder() const              { return m_data.runOrder; }
     uint32_t Config::rngSeed() const                   { return m_data.rngSeed; }
+    unsigned int Config::shardCount() const            { return m_data.shardCount; }
+    unsigned int Config::shardIndex() const            { return m_data.shardIndex; }
     UseColour Config::useColour() const                { return m_data.useColour; }
     bool Config::shouldDebugBreak() const              { return m_data.shouldDebugBreak; }
     int Config::abortAfter() const                     { return m_data.abortAfter; }