feat: use a command to get the API token

[UtkarshVerma]

Is your feature request related to a problem? Please describe.
Currently, the plugin stores the API token as plain text in .obsidian/todoist-token which is not a good practice. Since I use a password manager (rbw) that presents a CLI for accessing the secrets, I would like to have the plugin ask rbw for the API token instead.

Describe the solution you'd like
We can have a configuration option that allows specifying the command to fetch the API token. In my case, that would be rbw get api-keys/todoist.

Describe alternatives you've considered
N/A

Additional context
Other applications handle this problem in the same way.

[jamiebrynes7]

This is a great idea - I'm not sure how/if it could work on mobile platforms, but it seems sensible to allow this on desktop ones as an option!