Relay Agent helper: add support for source, hostname, and network instance

[rachidtt]

This is a follow-up and update to #964 #951 . More context can be found there

Change Scope

• Replace “helper-address” leaf-list with keyed list
• Update address type to be oc:inet:host to support ip and hostname configuration
• Add support for source-interface or source-address configuration
• Add option to specify network-instance
• This change is backwards compatible

Platform Implementations

• Arista: ip helper-address

New tree (relevant changes):

  +--rw relay-agent
     +--rw dhcp
     |  +--rw interfaces
     |     +--rw interface* [id]
     |        +--rw id                          -> ../config/id
     |        +--rw config
     |        |  +--rw id?                 oc-if:interface-id
     |        |  +--rw enable?             boolean
     |        |  x--rw helper-address*     inet:ip-address
     |        |  +--rw helper-addresses
     |        |     +--rw helper-address* [address]
     |        |        +--rw address    -> ../config/address
     |        |        +--rw config
     |        |        |  +--rw address?            oc-inet:host
     |        |        |  +--rw local-address?      union
     |        |        |  +--rw network-instance?   oc-ni:network-instance-ref
     |        |        +--ro state
     |        |           +--ro address?            oc-inet:host
     |        |           +--ro local-address?      union
     |        |           +--ro network-instance?   oc-ni:network-instance-ref
     |        +--ro state
     |        |  +--ro id?                 oc-if:interface-id
     |        |  +--ro enable?             boolean
     |        |  x--ro helper-address*     inet:ip-address
     |        |  +--ro helper-addresses
     |        |  |  +--ro helper-address* [address]
     |        |  |     +--ro address    -> ../config/address
     |        |  |     +--ro config
     |        |  |     |  +--ro address?            oc-inet:host
     |        |  |     |  +--ro local-address?      union
     |        |  |     |  +--ro network-instance?   oc-ni:network-instance-ref
     |        |  |     +--ro state
     |        |  |        +--ro address?            oc-inet:host
     |        |  |        +--ro local-address?      union
     |        |  |        +--ro network-instance?   oc-ni:network-instance-ref
     +--rw dhcpv6
        +--rw interfaces
           +--rw interface* [id]
              +--rw id               -> ../config/id
              +--rw config
              |  +--rw id?                 oc-if:interface-id
              |  +--rw enable?             boolean
              |  x--rw helper-address*     inet:ipv6-address
              |  +--rw helper-addresses
              |     +--rw helper-address* [address]
              |        +--rw address    -> ../config/address
              |        +--rw config
              |        |  +--rw address?            oc-inet:host
              |        |  +--rw local-address?      union
              |        |  +--rw network-instance?   oc-ni:network-instance-ref
              |        +--ro state
              |           +--ro address?            oc-inet:host
              |           +--ro local-address?      union
              |           +--ro network-instance?   oc-ni:network-instance-ref
              +--ro state
              |  +--ro id?                 oc-if:interface-id
              |  +--ro enable?             boolean
              |  x--ro helper-address*     inet:ipv6-address
              |  +--ro helper-addresses
              |  |  +--ro helper-address* [address]
              |  |     +--ro address    -> ../config/address
              |  |     +--ro config
              |  |     |  +--ro address?            oc-inet:host
              |  |     |  +--ro local-address?      union
              |  |     |  +--ro network-instance?   oc-ni:network-instance-ref
              |  |     +--ro state
              |  |        +--ro address?            oc-inet:host
              |  |        +--ro local-address?      union
              |  |        +--ro network-instance?   oc-ni:network-instance-ref```

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.