Wrap PSR-20 clock

Is your feature request related to a problem?
Our application defines a PSR-20 Clock and wants all dependent "current time" to be based off of it.
I'm aware that opentelemetry requires nanosecond precision which PSR-20 does not define.
However the SystemClock current implementation does 2 things:

1. Get a reference time using microtime() + hrtime()
2. Compute a current time using this reference time and a new call to hrtime().

Describe the solution you'd like
I'd love to see a bundled clock (we currently just implement our own) that looks like this:

<?php

declare(strict_types=1);

namespace collecthor\components;

use DateTimeImmutable;
use OpenTelemetry\API\Common\Time\ClockInterface as OtelClockInterface;
use Psr\Clock\ClockInterface;

final class OtelClock implements OtelClockInterface
{
    private readonly int $referenceTime;

    public function __construct(DateTimeImmutable|ClockInterface $reference)
    {
        $now = $reference instanceof ClockInterface ? $reference->now() : $reference;
        $this->referenceTime = $now->getTimestamp() * OtelClockInterface::NANOS_PER_SECOND
            + (int) $now->format('u') * OtelClockInterface::NANOS_PER_MICROSECOND;
    }

    public function now(): int
    {
        return $this->referenceTime + hrtime(true);
    }
}

One of the goals of PSR-20 is to make a dependency on time explicit. This solution provides that benefit while still conforming to the requirements of opentelemetry of nanosecond precision.

Additional context
This is not a duplicate of https://github.com/open-telemetry/opentelemetry-php/issues/950.

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Start with the existing SystemClock implementation and its current-time integration points, then compare them with the proposed PSR-20-compatible clock shape. Done means a bundled clock can use a DateTimeImmutable or PSR-20 ClockInterface as its reference while retaining the nanosecond precision required by OpenTelemetry.