Add Sixt car-share provider to providers.csv

[bruno-cardenas]

---

name: Default
about: Suggest changes to MDS
title: Add Sixt as a provider

---

MDS Pull Request

Add Sixt as a provider

Explain pull request

Please provide a clear and concise reason for this pull request and the impact of the change

Is this a breaking change

• No, not breaking

Impacted Spec

• provider

Additional context

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[schnuerle]

This looks good thank you! Make sure to sign the agreement: https://cla-assistant.io/openmobilityfoundation/mobility-data-specification?pullRequest=958

[bruno-cardenas]

Thank you for reviewing @schnuerle!
I'm checking the agreement and once confirmed I will mark the PR ready for review, thanks.