feat(cmd/influxd): add print-config subcommand to support automation

[danxmoran]

Closes #20517

This command is meant to support our Docker entry-point in two ways.

First, in our documentation for how to customize the influxd config we can advise running influxd print-config to generate a starting template file. This mirrors the existing docs for our 1.x image.

Second, we want the container entry-point to support running setup scripts mounted by the user, but only if the DB hasn't already been set up. To do this in a robust way while also supporting custom config / env vars / CLI args, we need to be able to determine the final values of configs like the bolt path & HTTP bind address. We can use influxd print-config with the --key-name option to do this.

If the user runs something like:

docker run -e INFLUXD_OPT2=val2 influxdb:2.0 --opt1 val1

Then within our entrypoint script we can run:

opt1=$(influxd print-config --key-name opt1 ${@}) # Expected output: val1
opt2=$(influxd print-config --key-name opt2 ${@}) # Expected output: val2

[lesam]

Where does the config file read happen in this code path (I'm sure it does, looking to be educated)

[danxmoran]

cli.NewCommand (called here) is where config & env get loaded into the Viper instance. cli.BindOptions (called here) is where the CLI flags get bound to local vars, with any config/env values injected as defaults.

[lesam]

lgtm