zebra: add redistribute table-direct support

zebra: add redistribute table-direct support

Redistributing routes from a specific routing table to a particular routing
protocol necessitates copying route entries to the main routing table using the
"ip import-table" command. Once copied, these routes are assigned a distinct
"table" route type, which the "redistribute table" command of the routing
protocol then picks up.

For illustration, here is a configuration that showcases the use of
"import-table" and "redistribute":

> # show running-config
> [..]
> ip route 172.31.0.10/32 172.31.1.10 table 100
> router bgp 65500
>  address-family ipv4 unicast
>   redistribute table 100
>  exit-address-family
> exit
> ip import-table 100
>
> # show ip route vrf default
> [..]
> T[100]>* 172.31.0.10/32 [15/0] via 172.31.1.10, r2-eth1, weight 1, 00:00:05

However, this method has inherent constraints:

- The 'import-table' parameter only handles route table id up to 252. The
253/254/255 table ids are reserved in the linux system, and adding other table
IDs above 255 leads to a design issue, where the size of some tables is directly
related to the maximum number of table ids to support.
- Duplicated route entries might interfere with original default table routes,
leading to potential conflicts. There is no guarantee that the zebra RIB will
favor these duplicated entries during redistribution.
- There are cases where the table ID can be checked independently of the default
routing table, as seen in Linux where the "ip rule" command is able to divert
traffic to that routing table. In that case, there is no need to duplicate route
entries in the default routing table.

To overcome these issues, a new redistribution type is proposed to redistribute
route entries directly from a specified routing table, eliminating the need for
an initial import into the default table.

Add a 'ZEBRA_ROUTE_TABLE_DIRECT' type to the 'REDISTRIBUTE' ZAPI messages. It
allows sending routes from a given non default table ID from zebra to a routing
daemon. The destination routing protocol table must be the default table.
The redistributed route inherit from the default distance value of 14: this is
the distance value reserved for routes redistributed via ROUTE_TABLE_DIRECT.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>