API key validation

[ob-stripe]

r? @remi-stripe

Validates API keys that are passed to StripeConfiguration.ApiKey = key;, new SomeService(key) and new RequestOptions { ApiKey = key }. If the key contains invalid characters (anything other than alphanumeric characters or underscores), an exception is raised.

Partially fixes #1507.

[remi-stripe]

@ob-stripe I don't think we should limit to alphanumeric and underscore. The current ongoing project internally actively considered adding a dash or another character to the API key. We ended up not doing it, but we might in the future and we don't want stripe-dotnet to be extremely strict on that front.

[ob-stripe]

I've updated the validation to only check for whitespace characters. ptal @remi-stripe

[remi-stripe]

Minor comments but feel free to self-approve/merge

[remi-stripe]

@ob-stripe Added a comment, feel free to self-approve

[ob-stripe]

@remi-stripe Sorry for the back and forth here, but mind taking another look at this PR? I've updated it to be consistent with what we do in stripe-ruby (https://github.com/stripe/stripe-ruby/blob/65cc6998329cfff8c7ae3df1090116fd6764699a/lib/stripe/stripe_client.rb#L236-L251). I think it's better this way since we only need to check the key in one place.