add clear command to cli

[corylanou]

This command adds the ability to clear out settings in the cli such as database or retention policy. It can be expanded to clear other settings if needed in the future.

This become more of an issue once we introduced the ability to set the retention policy context in #7708

example usage:

> clear
Possible commands for 'clear' are:
    # Clear the database context
    clear database
    clear db

    # Clear the retention policy context
    clear retention policy
    clear rp
> use foo
Using database foo
> settings
Host                                                                                                            localhost:8086
Username
Database                                                                                foo
Pretty                                                                                          false
Format                                                                                          column
Write Consistency       all

> clear database
database context cleared
> settings
Host                                                                                                            localhost:8086
Username
Database
Pretty                                                                                          false
Format                                                                                          column
Write Consistency       all

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated

Required only if applicable

• InfluxData Documentation: add updated usage for use and new clear commands in cli docs.influxdata.com-ARCHIVE#923

[e-dard]

Is clear database much different from use anotherDatabase?

What about if we just have a clear that can reset everything, as if you exited influx and started it again?

Just some thoughts. Otherwise LGTM 👍