[FEAT] Clarify caCertificate in documentation

[nferch]

The documentation for caCertificate should be expanded to be less ambiguous, especially compared to other systems with a similar parameter. Specifically: when configuring for the first time, I was left wondering:

• Why does caCertificate need to be Base64 encoded when certificates are already PEM encoded? It is unusual to require Base64 encoding for data that couldn't be binary. The other configuration parameters don't require Base64.
• Is caCertificate always required? Or is it only required when I have my own CA? If I'm using a standard CA, does the action make use of the standard CA bundle?

[WoodyWoodsta]

@nferch Do you know the answers to these? :) Specifically the second question. I have vault exposed behind a proxy using a letsencrypt certificate. My assumption is that this known CA should already be trusted.

[fairclothjm]

@nferch @WoodyWoodsta Hello! vault-action uses the got library. The vault-action caCertificate field is used to set got's https.certificateAuthority option. This Overrides trusted CA certificates and defaults to CAs provided by Mozilla. So it is not required and the table here documents it as such.

In regards to base64 encoding, I don't have a great answer. It may have been done that way due to limitations of GitHub's core action library. But I don't know for sure.