Accept priority as string

**Is your feature request related to a problem? Please describe.**
Home-Assistant notify service with POST_JSON type always sends priority as sting in resulting JSON due to templating engine (similar how ansible jinja templating works), that is not accepted by gotify server with 400 bad requests, as it accepts strict integer value for priority only

**Describe the solution you'd like**
gotify server to accept priority as string, and throw an error only if string value is not castable to integer

**Describe alternatives you've considered**
No real alernativies found so far, only hardcoding particular value/default priority which doesn't serve the purpose of prioirty differentiation within single app messages

**Additional context**
N/A