[API] New api in IMethodBinding to get the method JVM method signature

Today the jvm method signature is part of the `getKey` method. But it is not guaranteed as it seems and its not the method signature only.

When using AST for java debug breakpoint evaluations, we need to get the method signature from the AST method bindings so it could be matched later with JDI method signatures.

One usage case for this API is searching for method declaration of the given line number and use the signature of that method to fine JDI locations in vscode java debugger plugin.