Added github templates for issues and PRs

.github/ISSUE_TEMPLATE.md

+**Magento version**: x.y.z
+**Plugin version**: x.y.z
+**Description**
+<!--
+- please provide description of the issue. In case of bug report, please provide the necessary steps to reproduce.
+- For merchant specific requests, please use https://support.adyen.com
+-->
\ No newline at end of file

.github/PULL_REQUEST_TEMPLATE.md

+**Description**
+<!-- Please provide a description of the changes proposed in the Pull Request -->
+
+**Tested scenarios**
+<!-- Description of tested scenarios -->
+
+**Fixed issue**:  <!-- #-prefixed issue number -->
\ No newline at end of file