CPLLoadConfigOptionsFromFile(): add a [credentials] section to load VSI credentials

[rouault]

Extract from doc

Starting with GDAL 3.5, credentials can be specified in a
:ref:`GDAL configuration file <gdal_configuration_file>`, either in a specific one
explicitly loaded with :cpp:func:`CPLLoadConfigOptionsFromFile`, or
one of the default automatically loaded by :cpp:func:`CPLLoadConfigOptionsFromPredefinedFiles`.

They should be put under a ``[credentials]`` section, and for each path prefix,
under a relative subsection whose name starts with ``[.`` (e.g. ``[.some_arbitrary_name]``),
and whose first key is ``path``.
`
.. code-block::

    [credentials]

    [.private_bucket]
    path=/vsis3/my_private_bucket
    AWS_SECRET_ACCESS_KEY=...
    AWS_ACCESS_KEY_ID=...

    [.sentinel_s2_l1c]
    path=/vsis3/sentinel-s2-l1c
    AWS_REQUEST_PAYER=requester