[a11y][Obs Alert Rules] Keyboard focusable

[rshen91]

Summary

Closes https://github.com/elastic/observability-dev/issues/3373 and https://github.com/elastic/observability-dev/issues/3360

Adding the check viewInAppUrl !== '' allows the More Action Alerts Cell to be navigatable by keyboard. The user can now arrow into the cell and click enter to interact with the actions in the cell. The href prop was changed to an onClick because the href was blocking the keyboard accessibility.