feature: add supprot for external | url method when create coveragestore

[maslke]

Solved #22 : add support for external | url method in method create_coveragestore.

now we can use like this:

geoserver.create_coveragestore(
        path="http://localhost:8081/ECMWF_ERA-40_subset.nc",
        workspace="test",
        layer_name="url_netcdf",
        file_type="NetCDF",
        content_type="application/x-netcdf",
        method="external",
    )

[iamtekson]

@maslke, thank you very much for your contribution. It looks good with minor edits. I will accept the PR once it is updated.

[iboates]

Please also consider writing tests to cover main functionality

[maslke]

I've optimized the code. Please take a look when it's convenient for you.

[iamtekson]

LGTM. Thanks @maslke for your contribution.