Add support for (optional) organization & version for model selection in Get request

[b-eyselein]

Currently you can only specify a (comma separated) list of names for the models that should be used when performing a get request. Open execution the first model with the same name (provided by a capabilities request) is used, regardless of version and organization.

This pull requests adds the possibility to further select the right models for a get request by optionally specifying the organization (by prepending <organization>/) or the required version (by appending :<version>). The current functionality is not changed but rather extended.

[google-cla]

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[karimra]

Thanks for the contribution!