fix: import keys with a default key id

[rzana]

Related issue

#1741

When importing keys without kid attribute:

hydra keys delete hydra.jwt.access-token
hydra keys import hydra.jwt.access-token private.key public.key

the kids for the keys are created in a format of private:uuid1 and public:uuid2. Trying to create a token using that causes the following error:

public and private key pair kids do not match

Proposed changes

I added a default-key-id param to NewKeysImportCmd as a fallback kid value, respecting the previous behavior as well.

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• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
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• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

[CLAassistant]

All committers have signed the CLA.

[rzana]

If the general idea is acceptable I can create tests for the change.

[aeneasr]

Looking good! :)

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[aeneasr]

Thank you! :)