Support Vault Credential Library Mapping Overrides

[ddebko]

Summary:

• support using username_attribute & password_attribute for vault credential library that is a username_password credential type
• support using username_attribute, private_key_attribute & private_key_passphrase_attribute for vault credential library that is a ssh_private_key credential type

Open Questions:

when using the cli to remove an existing mapping override from a vault credential, you need to set the attribute value to null. Example:

boundary credential-libraries update vault \
    -id clvlt_WC6cZR7gKG \
    -credential-mapping-override "username_attribute=null" \
    -credential-mapping-override "password_attribute=null"

I thought that it might be weird to set an attribute value to null in the terraform resource. I think people would naturally try to just remove the resource credential_mapping_overrides or an attribute within the map. So I added logic on the update function to handle setting a removed value from the state file to nil for the user.

[louisruch]

This is great thanks!