Add CODEOWNERS and PR template

.github/CODEOWNERS

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+# Automatically assign the team as a reviewer.
+# https://help.github.com/en/articles/about-code-owners
+
+# Default owners, overridden by file/directory specific owners below
+* @DataDog/dd-trace-cpp

.github/pull_request_template.md

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+# Description
+
+# Motivation
+
+# Additional Notes
+
+Jira ticket: [PROJ-IDENT]
+
+<!--
+# Opening vs Drafting a PR:
+When opening a pull request, please open it as a draft to not auto assign reviewers before you feel the pull request is in a reviewable state.
+
+# Linking a JIRA ticket:
+Please link your JIRA ticket by adding its identifier between brackets (ex [PROJ-IDENT]) in the PR description, not the title.
+This requirement only applies to Datadog employees.
+-->