feat: add getJson and outputJson for plugin io util

[nilslice]

Looks like zig@master changed some build APIs, so i'll figure that out and update this PR (and likely the SDK repo too)

[nilslice]

I don't know of a way to better handle this such that the end-user can get the automatic type from the JSON, and have enough control over freeing the memory so it doesn't deallocate before a value is used.

In the example here:

const Input = struct {
    name: []const u8,
    age: u16,
};

export fn json_input() i32 {
    const plugin = Plugin.init(allocator);
    const json = plugin.getJson(Input, .{}) catch unreachable; // you must call .deinit() when done
    defer json.deinit();

    const input: Input = json.value();
    const out = std.fmt.allocPrint(allocator, "Hello, {s}. You are {d} years old!\n", .{ input.name, input.age }) catch unreachable;

    plugin.output(out);
    return 0;
}

The input.name (a []const u8) would be freed unless json.deinit() is called after this function returns and the value is used outside this function. Initially I thought I could handle this for the end-user, but I don't think I can.

So, this returns the Json wrapped type, with methods .value() (to get the parsed struct), and .deinit() to give the end-user the ability to free the slice used to parse the struct.

[zshipko]

I think you can avoid this by using alloc_always when parsing the JSON:

std.json.parseFromSlice(Config, allocator, data, .{.allocate = .alloc_always})

[nilslice]

@zshipko thanks, this works great!

[nilslice]

ok, now there is a getJson which returns the comptime T and handles the memory management. kind of the "expected / easy-mode" path.

and, there's a getJsonOpt which is the previous implementation and allows the end user to control the parsing and allocation.

[zshipko]

Looks good!

[nilslice]

Going to update the README to match these changes (and today's updates to Zig 😆) and will merge.