Redis PUB/SUB (#21728)

### What does this PR do?

The goal of this PR is to introduce PUB/SUB functionality to the
built-in Redis client. Based on the fact that the current Redis API does
not appear to have compatibility with `io-redis` or `redis-node`, I've
decided to do away with existing APIs and API compatibility with these
existing libraries.

I have decided to base my implementation on the [`redis-node` pub/sub
API](https://github.com/redis/node-redis/blob/master/docs/pub-sub.md).



### How did you verify your code works?

I've written a set of unit tests to hopefully catch the major use-cases
of this feature. They all appear to pass:

<img width="368" height="71" alt="image"
src="https://github.com/user-attachments/assets/36527386-c8fe-47f6-b69a-a11d4b614fa0"
/>


#### Future Improvements

I would have a lot more confidence in our Redis implementation if we
tested it with a test suite running over a network which emulates a high
network failure rate. There are large amounts of edge cases that are
worthwhile to grab, but I think we can roll that out in a future PR.

### Future Tasks

- [ ] Tests over flaky network
- [ ] Use the custom private members over `_<member>`.

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Dylan Conway <dylan.conway567@gmail.com>
Co-authored-by: Alistair Smith <hi@alistair.sh>

src/string/StringBuilder.zig

@@ -236,6 +236,15 @@ pub fn writable(this: *StringBuilder) []u8 {
     return ptr[this.len..this.cap];
 }
 
+/// Transfer ownership of the underlying memory to a slice.
+///
+/// After calling this, you are responsible for freeing the underlying memory.
+/// This StringBuilder should not be used after calling this function.
+pub fn moveToSlice(this: *StringBuilder, into_slice: *[]u8) void {
+    into_slice.* = this.allocatedSlice();
+    this.* = .{};
+}
+
 const std = @import("std");
 const Allocator = std.mem.Allocator;