[IMPROVEMENT] Utilize char array for storing and handling the Vault token

[jon5477]

Is your feature request related to a problem? Please describe.
According to the official Java Cryptography Architecture, you should not use String for storing the Vault token but instead use char[] to safely and securely handle security-sensitive information.

This feature makes String objects unsuitable for storing security-sensitive information such as user passwords. You should always collect and store security sensitive information in a char array instead.

Describe the solution you'd like
Internal handling of the token as a char[] and passing directly to the HTTP header X-Vault-Token. I know the restriction is that the HTTP header code only accepts a String, for this case it is acceptable to use new String(...) and pass in the char[] token because the String will be short-lived and not passed to other areas of the application.

Describe alternatives you've considered
I know a lot of legacy code may utilize the String as a token, therefore I suggest utilizing CharSequence to maximize legacy code compatibility as people slowly transition to using char[]. I propose providing 2 methods on the VaultConfig class to maximize compatibility:

public void setToken(CharSequence token) {
// store token
}

public void setToken(char[] token) {
// store token
}

Additional context
I've made some of those changes on my forked branch already, you can view those changes here.