Add /api/{alive,now,version} endpoints

[jjlin]

The added endpoints work the same as in their upstream implementations.

Upstream also implements /api/ip. This seems to include the server's public
IP address (the one that should be hidden behind Cloudflare), which doesn't
seem like a great idea.

[BlackDex]

Yea, these are some nice additions. I agree on the IP endpoint. Even if you would hide the servers public endpoint, you could still figure out the forwarded-for IP's which might also be unwanted.

[GoliathLabs]

@jjlin @BlackDex would it be an option that you could switch whether /api/ip should expose the IP or not? Since most users are probably not behind cloudflare and therefore have no disadvantage in providing the IP endpoint

[hevisko]

Hmmm... What would /api/ip return if it's behind a cloudflare behind a caddy/nginx, ie. the vaultwarden's IP would be localhost, the caddy's IP is a rfc1918 IP, which sits behind a firewall that NATs the external IP to the caddy, that might an IP behind cloudflare, so perhaps making the IP a trinary setting would be great:, like:
0.0.0.0 - off, return 404
anything from 0.0.0.1 to 255.255.255.254 - the value given (might be bogus)
255.255.255.255 - the IP that vaultwarden is listening on