Describe the bug

When there is Vault backend within a composite backend, then it cannot do server-side authentication. For example, it cannot do AppRole-based authentication.

Given following configuration for config-server:

spring:
  profiles:
    active: composite
  cloud:
    config:
      server:
        composite:
          - type: vault
            host: my-vault-server
            port: 5555
            authentication: approle
            roleId: 123
            secretId: 456

It won't be able to authenticate with Vault using its AppRole's role id and secret id. Instead it throws an error, saying that it expects the X-Config-Token header. This tells us that it's using a wrong config token provider.

The reason is that a default session manager bean is being created with the default Vault properties. However, a session manager is needed which uses the Vault properties from the Vault in the composite backend.

Authored w/ @kvmw