URL filtering - advanced operators

[bouwe77]

Advanced Operators

Operators for structural evaluation and pattern matching. Invalid regex syntax must return a 400 Bad Request.

• [exists] - field is present or absent (?filter.email[exists]=true)
• [regex] - full regular-expression match (?filter.name[regex]=^A.*e$)

Add to the docs:

| `[exists]`     | field is present (`true`) or absent (`false`) | `?filter.email[exists]=true`                       |
| `[regex]`      | full regular-expression match (URL-encode)    | `?filter.name[regex]=^A.*e$` → `%5E%A.*e%24`       |