add /filter/validate endpoint

[tyrasd]

Description

Adds a new endpoint /filter/validate which tests a provided ohsome filter string for syntax correctness.

Examples:

Request: GET http://127.0.0.1:8083/filter/validate?filter=foo=bar
Response: HTTP 200, body: foo=bar

Request: GET http://127.0.0.1:8083/filter/validate
Response: HTTP 400, body: {…, "status": 400, message: "No filter parameter provided.", … }

Request: GET http://127.0.0.1:8083/filter/validate?filter=type:
Response: HTTP 400, body: {…, "status": 400, message: "Invalid filter syntax. … Detailed error message: line1, column 6: … expected, EOF encountered.", … }

Request: POST http://127.0.0.1:8083/filter/validate, body: filter=foo%3Dbar%26bar
Response: HTTP 200, body: foo=bar

Request: POST http://127.0.0.1:8083/filter/validate, body: <empty>
Response: HTTP 400, body: {…, "status": 400, message: "No filter parameter provided.", … }

Request: POST http://127.0.0.1:8083/filter/validate, body: filter=filter=type%3A
Response: HTTP 400, body: {…, "status": 400, message: "Invalid filter syntax. … Detailed error message: line1, column 6: … expected, EOF encountered.", … }

This endpoint is currently considered "experimental" and therefore not listed in the documentation or the swagger ui.

Checklist

• My code follows the code-style rules, and I have checked on the static analyses
• I have commented my code
• I have written javadoc (required for public methods)
• I have added sufficient unit and API tests
• I have made corresponding changes to the documentation
• I have updated the CHANGELOG.md
• I have adjusted the examples in the check-ohsome-api script or created an issue in the corresponding repository. More Information here.