Added support for "UNDERLAY_SET_DSCP" and "UNDERLAY_SET_DSCPV6" tables

[siqbal1986]

What I did
This PR adds the logical table UNDERLAY_SET_DSCP and UNDERLAY_SET_DSCPV6.
This feature allows SONIC to match an ingressing L4 packet and change the DSCP field of the outer header when the packet is egressing.
These tables are only created on Cisco-8000, Mlnx and VS platforms.
Why I did it
The match set for these tables are following. The action for both of these tables is SET_DSCP
UNDERLAY_SET_DSCP

• SRC_IP
• DST_IP
• IP_PROTOCOL
• L4_SRC_PORT
• L4_DST_PORT
• TCP_FLAGS
• DSCP

UNDERLAY_SET_DSCPV6

• SRC_IPV6
• DST_IPV6
• IPV6_NEXT_HEADER
• L4_SRC_PORT
• L4_DST_PORT
• DSCP
  Note: These tables are not created but only their names are used.

Since we require matching based on L4 parameters, all the mentioned match attributes are necessary. Merging these into a single table would result in a larger TCAM footprint which may be impact existing ACL usage scenarios.

These table names transalte into MARK_META and MARK_METAV6 and EGR_SET_DSCP tables.
The translation is such that an attempt to create UNDERLAY_SET_DSCP or UNDERLAY_SET_DSCPV6 results into creation of
MARK_META & EGR_SET_DSCP or MARK_METAV6 & EGR_SET_DSCP .
The EGR_SET_DSCP table is shared and created only once. if both UNDERLAY_SET_DSCP and UNDERLAY_SET_DSCPV6 are created then only one instance of EGR_SET_DSCP is created to save tcam resource. The EGR_SET_DSCP is created in the Egress stage.

These internal MARK_META/V6 and EGR_SET_DSCP tables use SAI_ACL_ENTRY_ATTR_ACTION_SET_ACL_META_DATA and SAI_ACL_ENTRY_ATTR_FIELD_ACL_USER_META to set and match the metaData field.
For Example:
if the intent is to match an ingressing packet with fields
"DST_IP": "20.0.0.1/32",
"SRC_IP": "10.0.0.0/32",
"DSCP": "1"
"SRC_PORT" : "10"
"DST_PORT" : "20"
and set its outer DSCP to 12 after encapsulation,
This would be done so by MARK_META entry in ingress tcam matching based on above mentioned critera and setting a metaData value e.g. "1"
Another EGR_SET_DSCP entry in egress stage would be created with match criteria of metaData=1 and action of SET_DSCP=12.

Each entry in the EGR_SET_DSCP table is refcounted and shared among all the rules interested in same DSCP value.
There are 7 metadata values available [1...7]. This is currently hardcoded but this can be enhanced based on capibility check.

A Metadata Manager class governs the allcoation and freeing of each metadata value and they are shared based on DSCP value.
How I verified it
Added 16 Tests in sonic-mgmt. All passing.

Manual Test:
Tests added for the tables and Metadata Manager. 9 Test cases have been added which verfiy the table creation, deletion, EGR_SET_DSCP referencing, Entry creation/deletion, metadata value allcation/ release and exhausion scenarios.

Details if related
tested on mlnx 2700 with following rule
{ "ACL_RULE": {
"OVERLAY_MARK_META_TEST|RULE0": {
"PRIORITY": "999",
"DSCP_ACTION": "40",
"SRC_IP": "1.1.1.1/32"
},
"OVERLAY_MARK_META_TEST|RULE1": {
"PRIORITY": "998",
"DSCP_ACTION": "42",
"SRC_IP": "2.2.2.2/32"
}
} and a vxlan tunnel