feat: add pr template (#2011)

Signed-off-by: Moritz Johner <beller.moritz@googlemail.com>

.github/pull_request_template.md

@@ -0,0 +1,19 @@
+## Problem Statement
+
+What is the problem you're trying to solve?
+
+## Related Issue
+
+Fixes #...
+
+## Proposed Changes
+
+How do you like to solve the issue and why?
+
+## Checklist
+
+- [ ] I have read the [contribution guidelines](https://external-secrets.io/latest/contributing/process/#submitting-a-pull-request)
+- [ ] All commits are signed with `git commit --signoff`
+- [ ] My changes have reasonable test coverage
+- [ ] All tests pass with `make test`
+- [ ] I ensured my PR is ready for review with `make reviewable`