Support defining file credentials as SecretString

[sebhopley]

Hi, I am proposing this change to resolve issue 131.

This changes allows someone to set a SecretString (in UTF8) as the secret file content instead of a SecretBinary.

I've provided a test for this, please let me know if you need additional testing.

Seb

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[chriskilding]

An update: I would like the dual string-or-secretbytes supplier in CredentialFactory to be used only with the Secret File type, as the other credential types don't need this behaviour. The implementation currently enables this behaviour for all credential types. Would you be able to create a separate StringOrSecretBytesSupplier inner static class within CredentialFactory that implements this behaviour, and pass it to the constructor of AwsFileCredentials?

[sebhopley]

Hi, sorry for the late response, yeah I'll make that change

[sebhopley]

So I went and looked at the code again, for both cases where SecretBytesSupplier is used we are hoping to use string secrets (in a similar way to the ssh key secret). Going to the conversation in the linked issue #131 I feel like the suggestion there to catch a failure to use UTF8 encoding and throw a descriptive error seems better than blanket not allowing this. Also as talked about, enforcing using the command line can be problematic (and means a user needs access keys) vs using the aws console.

Open to suggestions on what you would like to see to go forward with the change.

[chriskilding]

By both cases, do you mean the Certificate credentials type as well? As far as I know the certificate's keystore is supplied by a PKCS#12 (.p12) file, which is exclusively a binary file format.