Add support for 'influxd' configuration files

[sanderson]

PR: influxdata/influxdb#18568

influxd now supports configuration files. Some important things to cover:

• Configuration setting order of precendence:
  1. CLI flags
  2. Environment variables
  3. Settings defined in the config file
• Syntax and structure of the config file (yaml, toml, json)
• All the configuration keys (same as the CLI flags without the -- prefix)
• A config file can be provided via the INFLUXD_CONFIG_FILE env var. If a file is not provided via an env var, influxd will look in the current directory for a config.yaml file. If one does not exist, then it will continue unchanged.

Relevant URLs

• https://v2.docs.influxdata.com/v2.0/reference/config-options/
• https://v2.docs.influxdata.com/v2.0/reference/cli/influxd/