Use a covariant type for detectors argument (#5135)

This updates the type annotation for `get_aggregated_resources`'s
`detectors` argument to use `collections.abc.Sequence`, which is
covariant in its type parameter and thus has the expected behaviour for
an input type.

`typing.List` is invariant in its type parameter, which means that a
simple list of `ResourceDetector` subclasses is not a
`list[ResourceDetector]`.

Some typecheckers are more lenient than others, but [ty] enforces this
and thus the following code does not typecheck despite being
semantically correct.

    from opentelemetry.sdk.resources import (
        OsResourceDetector,
        ProcessResourceDetector,
        Resource,
        get_aggregated_resources,
    )
    from opentelemetry.sdk.trace import TracerProvider

    resource = Resource.create()

    resource_detectors = [
        ProcessResourceDetector(),
        OsResourceDetector(),
    ]

    tracer_provider = TracerProvider(
        resource=get_aggregated_resources(
            detectors=resource_detectors,
            initial_resource=resource,
        ),
    )

The full output of ty is as follows:

    $ uv run ty check
    error[invalid-argument-type]: Argument to function `get_aggregated_resources` is incorrect
      --> test.py:18:9
       |
    18 |         detectors=resource_detectors,
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Expected `list[ResourceDetector]`, found `list[ProcessResourceDetector | OsResourceDetector]`
       |
    info: Function defined here
       --> .venv/lib/python3.13/site-packages/opentelemetry/sdk/resources/__init__.py:507:5
        |
    507 | def get_aggregated_resources(
        |     ^^^^^^^^^^^^^^^^^^^^^^^^
    508 |     detectors: typing.List["ResourceDetector"],
        |     ------------------------------------------ Parameter declared here
        |
    info: `list` is invariant in its type parameter
    info: Consider using the covariant supertype `collections.abc.Sequence`
    info: For more information, see https://docs.astral.sh/ty/reference/typing-faq/#invariant-generics

    Found 1 diagnostic

[ty]: https://docs.astral.sh/ty/reference/typing-faq/#invariant-generics

Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>