feat(common): add shared telemetry module for optional OpenTelemetry instrumentation

### What you would like to be added?

## Context

Following the discussion in #164, this issue tracks the implementation
of the shared foundation module for OpenTelemetry instrumentation across
all Kubeflow SDK clients.

## Proposed Location

`kubeflow/common/telemetry.py`

## Design

- `get_tracer(name)` — returns real OTel tracer or zero-overhead NoOp
- `SpanNames` — canonical span name constants for all SDK clients
- `SpanAttributes` — canonical attribute key constants
- `configure(exporter, endpoint)` — user-facing one-call setup
- `_NoOpTracer` / `_NoOpSpan` — zero overhead when opentelemetry-api
not installed

## Key Principles

- `opentelemetry-api` is optional — not in core dependencies
- Zero overhead when not installed — NoOp with empty method bodies
- Polling loops: per-iteration child spans with span events
- Single source of truth for naming across all SDK clients

## Span Hierarchy
kubeflow.sdk.trainer.train
├── kubeflow.sdk.trainer.get_runtime
├── kubeflow.sdk.trainer.create_trainjob
└── kubeflow.sdk.trainer.poll_status [× N iterations]
events: status_check, job_reached_expected_status

Related to #164

### Why is this needed?

The Kubeflow SDK currently has no observability into its internal operations. When `TrainerClient.train()` runs, users cannot see runtime resolution, CRD submission, or polling behavior. Debugging failures and measuring latency requires digging into raw Kubernetes events.

A shared `telemetry.py` module in `kubeflow/common/` provides the foundation for all SDK clients to emit OTel traces without duplicating instrumentation code, and without any overhead for users who don't enable tracing.

### Love this feature?

Give it a 👍 We prioritize the features with most 👍

Start by reading the related discussion in #164 and the proposed kubeflow/common/telemetry.py location. Define the shared tracer, span-name and attribute constants, configuration entry point, and no-op behavior described in the issue; done means optional OpenTelemetry support works without core dependencies or overhead when disabled.

Written by the indexing model from the issue text.