feat: support for exit zero sigterm

[nancynh]

Adds a new flag exit-zero-on-sigterm to exit with a 0 exit code when SIGTERM is received. This is a port of CloudSQL's exit-on-zero-sigterm (GoogleCloudPlatform/cloud-sql-proxy#1870). Likewise, it is needed for applications that think that an exit code 143 is an error when it is not, and this provides users an easy workaround.

Fixes #683

[jackwotherspoon]

@enocom @nancynh I wonder if we should just add full support and feature parity for sigterm while you are it.

i.e. support for --min-sigterm-delay and --max-sigterm-delay

https://github.com/GoogleCloudPlatform/cloud-sql-proxy/blob/4cfa6a20763c483427331fae8e3227a64bc3b4a4/cmd/root.go#L466-L469

[enocom]

Yes, we probably should. I have separate tracking issue for min-sigterm-delay here: #670.

@nancynh could I interest you in picking that up? ^^;

[nancynh]

Yup, I can port both of those flags too in a separate PR