feat: add keyGet3 function

[AlabLan]

fix: #523

Summary

Added the keyGet3 function to jcasbin for the {var} placeholder style, which was previously only available in the Go version.

Problem

The documentation at https://casbin.apache.org/docs/function shows keyGet3 function, but jcasbin only implemented keyGet and keyGet2.

Implementation

• Added KeyGet3Func wrapper class
• Added keyGet3Func implementation in BuiltInFunctions
• Registered keyGet3 in FunctionMap
• Added comprehensive tests ported from Go's TestKeyGet3

Note on Go/Java Regex Incompatibility

The following Go test cases fail in Java due to regex engine differences:

• testKeyGet3("/{id/using/myresid", "/{id/using/{resId}", "resId", "myresid")
• testKeyGet3("/{id/using/myresid/status}", "/{id/using/{resId}/status}", "resId", "myresid")
• testKeyGet3("/api/group1_group_name/project1_admin/info", "/api/{proj}_admin/info", "proj", "")

Java treats { as a repetition quantifier modifier, so patterns like "/{id/using/(" are invalid. The Java implementation adds escaping for remaining { and } characters after the key replacement to
handle this incompatibility.