Avoid confliction of subscription names for multiple tenants

[yush1ga]

A topic of Pulsar can be used by multiple tenants.
They have to be careful not to make a subscription name conflict.

I propose subscription_auth_mode for namespace policies.

0. None

as before

1. Subscription Prefix

When subscription_auth_mode in a namespace policy is Prefix and AuthenticationEnabled in broker.conf is true, a client have to use subscription name including role name prefix like ${AuthRroleName}-foobar.

2. Reserved subscription name

When subscription_auth_mode is Reservation, refer to subscription_auth in AuthPolicies like following.

subscription_auth: {
    sub1: [role1],
    sub23: [role2, role3]
}

In this case, sub1 can be used by only role1 and sub23 can be used by only role2and role3.

[sijie]

This is fixed by #899