/vsicloud support for passing config options

[pramsey]

In order to access secured buckets in cloud objects, we need a way to pass in the various GDAL config options that allow security info to be added to requests.

It's possible to add in environment variables when starting a back-end but that will be waaaay too coarse for most use cases.

This ticket will add a per-session way of setting options that will be handed into each GDALOpenEx() call.

Client API will just be (for example in S3):

SELECT ST_GDALOpenOptions(ARRAY[
  'AWS_ACCESS_KEY_ID=xxxx', 
  'AWS_SECRET_ACCESS_KEY=yyyy'])

Those tokens will be passed to opens until the connection is closed, and they can be changed by the user at any time with another call to ST_GDALOpenOptions().

[Paul Ramsey <pramsey@…>]

In d758f50/git:

Allow open options to be passed to GDAL drivers (in particular the /vsi drivers for cloud buckets. Closes #4870