sdtype_adapter refactor (#1753)

* reorganize sampler selection in sdtype_adapter

* simplify image metadata generation

Use a stringstream to avoid the explicit conversions to
string. Also, generate from the sd.cpp struct, to reduce
dependencies on the global context fields.

* clean up default parameters in sdtype_adapter

Most image generation parameters are not customizable through
the API, and their values simply repeat the default set by
sd.cpp. So, remove them from the SDParams struct, and place
the needed constant values directly on the code.

Also, print debug information directly from the sd.cpp
parameters. And to help with type mismatches and general
readability, replace the fromatting with stringstreams.

Author: wbruna