remove sparql client dependency

[Rahien]

This removes the dependency on sparql-client-2, which is deprecated, unmaintained and uses the deprecated request library.

Instead, it uses digest-fetch to perform a regular fetch request (with support for sparql endpoints with authentication). This approach was taken from nvdks work on the mu-auth-sudo library: lblod/mu-auth-sudo#10

It also includes:

• the retry mechanism that was used in mu-auth-sudo, defaults to disabled
• bump of nodemon, which brings the number of npm audit warnings to 0
• the SPARQL template tag as it was defined by sparql-client-2

The exposed api of mu remains unchanged with this change.

[nvdk]

Huzaa! 🎆

[nvdk]

this fixes #69, just adding it here so the issue is linked

[erikap]

This PR needs an update to support the mu-auth-scope and mu-auth-sudo options introduced in 56e185e . This feature is currently available in master.