feat(telemetry): add register_sdk_resource_attributes() for companion SDKs

[jeanscherf]

Summary

• Adds register_sdk_resource_attributes(attributes: dict) to telemetry/config.py
• create_resource_attributes_from_env() merges registered attributes into the resource dict, so they appear on every span and metric
• Exports register_sdk_resource_attributes from sap_cloud_sdk.core.telemetry

Why

Companion internal SDKs (e.g. sap-internal-sdk) need a way to contribute their version as an OTel resource attribute without requiring agents to change their auto_instrument() calls. This PR provides the extension point.

Usage from a companion SDK at import time — version read dynamically from package metadata:

from importlib.metadata import version, PackageNotFoundError
from sap_cloud_sdk.core.telemetry.config import register_sdk_resource_attributes

try:
    sdk_version = version("my-sdk-package")
except PackageNotFoundError:
    sdk_version = "unknown"

register_sdk_resource_attributes({
    "sap.my_sdk.version": sdk_version,
    "sap.my_sdk.language": "python",
})

Test plan

• 4 new tests in TestRegisterSdkResourceAttributes covering registration, merging, and empty-registry no-op
• Full telemetry unit test suite: 404 passed