Erroneous ID/Tag parsing

[tbrezot]

          This holds for all actions:

• The current implemented introduces an undocumented behavior when both --id and --tag are passed. Using an enum could solve the problem:

struct AddAttributeAction {
    access_policy: String,
    handle: KeyHandle,
}

enum KeyHandle {
    Id(String),
    Tags(Vec<String>),
}

• The parsing of tags may be checked against the empty vector. It is not clear for me how the current implementation behaves when passed --tag with an empty string. My guess would be an "UnknownKeyId" error returned from a call to the KMS. This corner case could be checked locally instead.

Originally posted by @tbrezot in https://github.com/Cosmian/kms/pull/179#discussion_r1511330237