Plugins: be able to specify conda dependencies in pyproject.toml

[emlys]

Currently, all plugin dependencies are specificed in the pyproject.toml project.dependencies field, and must be installed by pip. We install python and gdal into the plugin's conda env for all plugins, assuming those will be needed. It would be better to let the plugin specify its conda dependencies (more explicit and allows plugins to have additional non-pure-python dependencies).

An open python PEP, PEP 725, would standardize a way to include conda dependencies in pyproject.toml. We might not use that syntax until the PEP is approved, but it might inform our design.