cli: add `show-effective-configuration` command

cli: add `show-effective-configuration` command

This new command is equivalent to `generate --dump-config`, but in a
more dedicated way:

    $ barectf show-effective-configuration config.yaml

It only accepts relevant options.

One of the options is `--indent-spaces` which controls the indentation
space count of printed YAML lines.

This patch deprecates `--dump-config`: I'll leave it working in
barectf 3 to remain backward-compatible, but I'm removing it from the
`generate` command's help.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>