feat(config): generic resource detector plugin loading for declarative config

[MikeGoldsmith]

Description

Extends resource detector support in declarative file configuration to handle custom (plugin) detectors, matching the spec's PluginComponentProvider mechanism.

Solution

_run_detectors() now iterates detector config dicts directly via _RESOURCE_DETECTOR_REGISTRY:

• Known names (service, host, process) are bootstrapped directly from the SDK
• Unknown names — including container and custom plugin detectors — are loaded via load_entry_point("opentelemetry_resource_detector", name)

The generated models are unchanged. Python dataclasses don't enforce field types at runtime, so the detectors list naturally accepts raw dicts (from the YAML loader) alongside typed ExperimentalResourceDetector instances. This preserves unknown plugin names as dict keys without diverging from codegen.

Behavior change

The container detector previously logged a warning when the contrib package was not installed. It now raises ConfigurationError, consistent with the fail-fast approach for declarative config (as discussed in the issue).

Custom detector example

resource:
  detection_development:
    detectors:
      - my_custom_detector: {}

[project.entry-points."opentelemetry_resource_detector"]
my_custom_detector = "my_package:MyResourceDetector"

Closes #5053