[Feature Request] Helper function to get the unique values from a list

[milas]

Feature description:
A to_unique() -> list (or equivalent) function to accompany the unique() -> bool CEL helper function.

Supported today:

[1, 1, 2, 3, 3 ].unique()      # false

Proposal:

[1, 1, 2, 3, 3 ].to_unique()   # [1, 2, 3]

.to_unique().unique() is always true.

Problem it solves or use case:
unique() is a helpful protovalidate addition for something you can't do in CEL otherwise. I want the intermediate form, i.e. the set of unique values, exposed to do even more!

Contribution:
I can likely contribute the PR for this - just let me know re: to_unique() naming at least if no other major concerns.

Examples or references:
Example: Ensure all items in a collection are all siblings

this.collection.map(x, x.parent_id).to_unique().size() == 1

Example: Ensure all structpb.Value items are of the same type

google.protobuf.Value value = 1 [
  (buf.validate.field).cel = {
    id: "repeated.same_type",
    message: "all values must be of same type",
    expression: "type(this) != list || this.map(v, type(v)).to_unique().size() <= 1"
                                          //               ^^^^^^^^^^^^^
                                          //             my hopes & dreams
  }
];

(can you guess this is the specific use case I have in mind? 😉)