Add Tailscale as a sidecar if enabled

[tancou]

Description of the change

Uptime Kuma natively supports pinging with the Tailscale VPN on the non-Dockerized version. I find it regrettable that we can't bridge with a Tailscale container in sidecar mode.
Therefore, I have added support, which can be enabled or not, with or without root, from the official Tailscale documentation in the chart.
The default values do not change the behavior of the chart.

Benefits

With a Tailscale device in sidecar mode, it is possible to access all the devices on the network directly via their IP, enabling all the monitoring supports that Uptime Kuma offers. Depending on the defined ACLs, it is also possible to access Uptime Kuma from the Tailscale network.

Possible drawbacks

To have direct access to the Tailscale network from Uptime Kuma without using the proxy option, the Tailscale in sidecar mode must run as root, which reduces security. See the documentation kubernetes#userspace-sidecar.

Root mode can be disabled via values.

Applicable issues

• fixes #

Additional information

Checklist

• Chart version bumped in Chart.yaml according to semver.
• Variables are documented in the README.md

[tancou]

I have just rebased my branch, bump the helm chart version to 2.20.0