Introduce "config edit-ini" sub command to help maintaining INI config file

[wxiaoguang]

Ref: #32669. Helps addressing https://gitea.com/gitea/helm-chart/issues/356.

• Keep Specified Keys

If you need to re-create the configuration file with only a subset of keys,
you can provide an INI template file and use the "--config-keep-keys" flag.
For example, if a helm chart needs to reset the settings and only keep SECRET_KEY,
it can use a template file like:

  [security]
  SECRET_KEY=

$ ./gitea config edit-ini --config app-old.ini --config-keep-keys app-template.ini --out app-new.ini

[lunny]

Maybe this is a good opportunity to restrict environment-based configuration to only specific sections.

[TheFox0x7]

Approving as the only concern I have are flag name and semi-clear instructions, the latter of which can easily be reworded if someone will have questions about the behavior.
I don't have much better ideas for the flag name though the template part still feels wrong here but naming things always was hard :)

[wxiaoguang]

Applied your suggestions and made new changes:

• use edit-ini instead of update-ini, "update" sounds like "change the existing thing", "edit" is like sed: edit and save
• add --in-place flag (also like sed), then we don't need to worry about the input file would be accidentally overwritten.
• use --config-keep-keys as you suggested.