Fixed dhcp Option 60 encoding

[gupurush]

Why I did it

• Fix Option 60 encoding.
	Update dhclient.conf.j2 templated to add "\0x00" as first character in the string to indicate that included id is a string
• Add support for Option 61
	To add vendor-class unconditionally in each message, updated the dhclient.conf.j2 to included vendor-name:product-name. The value for these variables is retrieved from eeprom. Product name is already available and added the option to retrieve vendor name thru decode-syseeprom utility.

Required PRs:
sonic-net/sonic-utilities#3765
sonic-net/sonic-platform-common#542

Logs:

# =============== Managed by SONiC Config Engine DO NOT EDIT! ===============
# generated from /usr/share/sonic/templates/dhclient.conf.j2 using sonic-cfggen
# file: /etc/dhcp/dhclient.conf
#
# Configuration file for /sbin/dhclient, which is included in Debian's
#       dhcp3-client package.
#
# This is a sample configuration file for dhclient. See dhclient.conf's
#       man page for more information about the syntax of this file
#       and a more comprehensive list of the parameters understood by
#       dhclient.
#
# Normally, if the DHCP server provides reasonable information and does
#       not leave anything out (like the domain name, for example), then
#       few changes must be made to this file, if any.
#

option rfc3442-classless-static-routes code 121 = array of unsigned integer 8;
option snmp-community code 224 = text;
option minigraph-url code 225 = text;
option acl-url code 226 = text;
option tftp-server-name code 66 = text;
option bootfile-name code 67 = text;
option user-class code 77 = text;
option provisioning-script-url code 239 = text;
option dhcp6.user-class code 15 = text;
option dhcp6.provisioning-script-url code 239 = text;
option dhcp6.boot-file-url code 59 = text;
option vendor-class code 60 = text;

send host-name = gethostname();
request subnet-mask, broadcast-address, time-offset, routers,
        domain-name, domain-name-servers, domain-search, host-name,
        dhcp6.name-servers, dhcp6.domain-search, interface-mtu, dhcp6.fqdn,
        rfc3442-classless-static-routes, ntp-servers, log-servers,snmp-community, minigraph-url, acl-url;

send vendor-class "Cisco:{MODEL-XXXX}"; 


18:31:56.604658 IP (tos 0x0, ttl 64, id 58732, offset 0, flags [DF], proto UDP (17), length 338)
    10.29.XXX.XXX.bootpc > hostname.example.com.bootps: [udp sum ok] BOOTP/DHCP, Request from 34:73:YY:YY:YY:YY (oui Unknown), length 310, xid 0xff8d9300, Flags [none] (0x0000)
          Client-IP 10.29.XXX.XXX
          Client-Ethernet-Address 34:73:YY:YY:YY:YY (oui Unknown)
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Release
            Server-ID Option 54, length 4: hostname.example.com
            Hostname Option 12, length 5: "sonic"
            Vendor-Class Option 60, length 18: "Cisco:{MODEL-XXXX}"
            Client-ID Option 61, length 31: "SONiC##MODEL-XXXX##XYZ" 
            
            
            
root@sonic:/home/cisco# ifconfig eth0
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet6 fe80::3673:2dff:fe30:70d8  prefixlen 64  scopeid 0x20<link>
        ether 34:73:YY:YY:YY:YY  txqueuelen 1000  (Ethernet)
        RX packets 9939  bytes 2618950 (2.4 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 1219  bytes 203002 (198.2 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device memory 0x95000000-9501ffff 
        ```

[mssonicbld]

/azp run Azure.sonic-buildimage

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[ashutosh-agrawal]

@yaqiangz Appreciate your help in reviewing this PR.