Add user tag prefix to in_forward plugin

### Is your feature request related to a problem? Please describe.

Hello,

We're trying to use forward plugin for multi-tenant configuration.
So each tenant would have it's own username and password.
It is basically described in [in_forward_users.conf](https://github.com/fluent/fluentd/blob/master/example/in_forward_users.conf) and [out_forward_users.conf](https://github.com/fluent/fluentd/blob/master/example/out_forward_users.conf) config examples.
What we also want - is to ensure that tenant1 won't send (audit) logs on behalf tenant2.
So, want to add `user_tag_prefix` to security section on server side (in_forward).

### Describe the solution you'd like

Current configuration ([in_forward_users.conf](https://github.com/fluent/fluentd/blob/master/example/in_forward_users.conf)):

```

@type forward

self_hostname input.testing.local
shared_key secure_communication_is_awesome
user_auth yes

username user1
password yes_this_is_user1


username user2
password yes_this_is_really_user2

```

Desired configuration:

```

@type forward

self_hostname input.testing.local
shared_key secure_communication_is_awesome
user_auth yes
user_tag_prefix yes #### NEW

username user1
password yes_this_is_user1


username user2
password yes_this_is_really_user2

```

As a result, tag would have prefix with username:

Original tag coming from user1: `auditlog`
Desired tag after `user_tag_prefix`: `user1.auditlog`

### Describe alternatives you've considered

Nothing

### Additional context

_No response_

Start with the in_forward plugin and the referenced example/in_forward_users.conf configuration, then trace where authenticated users and incoming tags are handled. The change is done when enabling user_tag_prefix makes user1's auditlog become user1.auditlog while preserving the existing authenticated forwarding behavior.

Written by the indexing model from the issue text.