[PROF-10241] Add extra "severity" tag to crash reports

**What does this PR do?**

This PR adds the "severity: crash" tag to crashtracker reports, similar
to what we do in Python in
DataDog/dd-trace-py#10199 .

In the future, this won't be needed by libdatadog, but for now this
makes it easier for us to analyze reported crashes.

**Motivation:**

Improve crash reports.

**Additional Notes:**

N/A

**How to test the change?**

Change includes test coverage.

lib/datadog/core/crashtracking/tag_builder.rb

@@ -23,7 +23,8 @@ def self.call(settings)
             'version' => settings.version,
             'git.repository_url' => Environment::Git.git_repository_url,
             'git.commit.sha' => Environment::Git.git_commit_sha,
-            'is_crash' => true
+            'is_crash' => 'true',
+            'severity' => 'crash',
           }.compact
 
           # Make sure everything is an utf-8 string, to avoid encoding issues in downstream

spec/datadog/core/crashtracking/tag_builder_spec.rb

@@ -16,6 +16,7 @@
         'runtime_platform' => RUBY_PLATFORM,
         'runtime_version' => RUBY_VERSION,
         'is_crash' => 'true',
+        'severity' => 'crash',
       )
     end