Support adding new OATH acccounts

[aripalo]

This is a placeholder issue for an optional feature that may be implemented in the future (if there's a need).

This is NOT a promise to implement!

But if you feel like it, here's some information on how to get started with the implementation. Feel free to create a pull request!

See also design decisions.

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The implementation in oath.go could be a method on the OathAccounts struct and look something like this:

func (oa *OathAccounts) Add(options AccountOptions) error {}

In the background it should implement ykman oath accounts add command with its various options.