gaiacli config: support --offline flag

[alessio]

Title is self-explanatory.
Acceptance criteria:

Given I installed gaiacli
When I run gaiacli config --get offline
Then I am presented with the value stored in ~/.gaiacli/config/config.toml.

Given I installed gaiacli
When I run gaiacli config offline true or gaiacli config offline false
Then the respective offline value is stored in ~/.gaiacli/config/config.toml.

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