Add standard lib functions

[isaiahvita]

This PR adds generic standard library functions for endpoints2.0.

An error/endpoints package was added in order to contain a utility ErrorCollector class that can provide detailed logging for errors encountered in rule evaluation. It was made public so that the additional SDK functions can depend on this as well.
The standard library functions were added in an internal package as required by the EP20 SEP.

EDIT: the core functions isSet not and getAttr will be implemented directly in codegen generation in a forthcoming PR. for example,

        @Override
        public GoWriter.Writable visitIsSet(Expr expr) {
            return (GoWriter w) -> {
                w.write("$W != nil", new ExprGenerator(scope).generate(expr));
            };
        }

        @Override
        public GoWriter.Writable visitNot(Expr expr) {
            return (GoWriter w) -> {
                w.write("!($W)", new ExprGenerator(scope).generate(expr));
            };
        }

        @Override
        public GoWriter.Writable visitBoolEquals(Expr left, Expr right) {
            return (GoWriter w) -> {
                var generator = new ExprGenerator(scope);
                w.write("$W == $W", generator.generate(left), generator.generate(right));
            };
        }

        @Override
        public GoWriter.Writable visitStringEquals(Expr left, Expr right) {
            return (GoWriter w) -> {
                var generator = new ExprGenerator(scope);
                w.write("$W == $W", generator.generate(left), generator.generate(right));
            };
        }

        @Override
        public GoWriter.Writable visitGetAttr(GetAttr getAttr) {
            var target = new ExprGenerator(scope).generate(getAttr.getTarget());
            var path = (GoWriter.Writable) (GoWriter w) -> {
                getAttr.getPath().stream().toList().forEach((part) -> {
                    if (part instanceof GetAttr.Part.Key) {
                        w.writeInline(".$L", getBuiltinMemberName(((GetAttr.Part.Key) part).key()));
                    } else if (part instanceof GetAttr.Part.Index) {
                        w.writeInline(".Get($L)", ((GetAttr.Part.Index) part).index());
                    }
                });
            };

            return (GoWriter w) -> w.writeInline("$W$W", target, path);
        }

        @Override
        public GoWriter.Writable visitLibraryFunction(FunctionDefinition fnDef, List<Expr> args) {
            return new FnGenerator(scope).generate(fnDef, args);
        }

[aajtodd]

Overall this looks really good. I have some concerns/questions around ErrorCollector and internal as well as some minor feedback.

One other question: I'm assuming that the other functions isSet, not, and getAttr will be implemented directly in codegen generation (i.e. inlined in codegen)?

[lucix-aws]

Approved contigent on addressing the internal/ issue as pointed out by Aaron.

[isaiahvita]

One other question: I'm assuming that the other functions isSet, not, and getAttr will be implemented directly in codegen generation (i.e. inlined in codegen)?

@aajtodd Good question, yah thats correct and i should have said this in the PR description. Will update in the PR description.