build: refactor, allow importing the package as a dependency

[inge4pres]

Reason for this PR

We intend to expose the package as a usable artifact for other Zig projects.

Closes #49

Details

There is a broader build.zig refactor that was needed to use proper build features.
Unrelated commit fixing a problem in the Histogram allowed value types.

Additional context

To test this out, you can zig init a new project, then

zig fetch --save https://github.com/zig-o11y/opentelemetry-sdk/archive/d285fc88d7b8cb5ca5469057d61c6e73a6fcf2b7.tar.gz

In the src/main.zig file just created, you can add calls to the OTel SDK that we now provide as a module, e.g.

...
    const mp = try otel.MeterProvider.default();
    defer mp.shutdown();

    const meter = try mp.getMeter(.{ .name = "test-meter" });
    const counter = try meter.createUpDownCounter(i64, .{
        .name = "test_counter",
        .description = "A counter for testing purposes",
    });
    const val: []const u8 = "Hello, OpenTelemetry!";
    try counter.add(1, .{ "test_key", val });

    const histogram = try meter.createHistogram(i64, .{
        .name = "test_histogram",
        .description = "A histogram for testing purposes",
    });
    try histogram.record(42, .{ "test_key", val });
...

and then run `zig build --verbose` to see it in action.