Add configuration to exit after retrieving JWTs (closes #121)

[keeganwitt]

[kfox1111]

Why two flags? There ever going to be a case where both jwt and x509 are enabled at the same time? If so, it probably should wait for both? If two flags, it may be confusing in that case.

[faisal-memon]

Im planning to do a small refactor to create a daemon mode flag. when disabled will just download the x509 and/or JWTs and then exit. That should cover this with only a single flag.

[keeganwitt]

Why two flags? There ever going to be a case where both jwt and x509 are enabled at the same time? If so, it probably should wait for both? If two flags, it may be confusing in that case.

Yes, I was thinking some use cases might require both. Ideally, yes, it would wait for both. But I wasn't sure how to implement that, particularly since there might be multiple JWTs. If someone can guide me with an idea, I'll give it a shot.

[faisal-memon]

@keeganwitt Finally got around to creating the PR i mentioned above with a single daemon_mode flag. Please take a look and lmk if its ok to close this PR. https://github.com/spiffe/spiffe-helper/pull/161

[keeganwitt]

Conceptually, yea, think that will solve what I was trying to do here.