enhance Klish parser with improved usability and python3 support

[kwangsuk]

Klish enhancements for sonic-mgmt-framework to improve usability and python3.7 support in Buster

1. Add two built-in methods for performance enhancement as part of mgmt-framework CLI enhancement:

CLI built-in functions "clish_pyobj" and "clish_restcl" can be use to reduce time taken to execute a command by eliminating the sub-shell interpreter overhead.

The “clish_restcl” method uses libcurl to make REST client call from actioner. The GET operation is not supported as we currently don't handle rendering using jinja templates.
e.g.
oper=PATCH url=/restconf/data/openconfig-interfaces:interfaces/interface=Vlan${vlan-id}/config body={"openconfig-interfaces:config": {"name": "Vlan${vlan-id}"}}

The “clish_pyobj” uses embedding Python to execute the Python scripts to make REST client call via Request package.
e.g.
sonic_cli_if patch_openconfig_interfaces_interfaces_interface_config Vlan${vlan-id}

2. New PTYPE attribute support - ext_pattern and ext_help with regexp_select method
   Similar to the pattern attribute with select method in Opensource Klish, the ext_pattern with regexp_select can be used for auto completion.
   e.g.

<PTYPE
name="PHY_INTERFACE"
method="regexp_select"
ext_pattern="Ethernet(port)"
pattern="&ETH_ALL_INTF;"
help="" />

An interface name can be specified as “interface Ethernet 1” or “interface Eth1”.

sonic-cli(config)# interface Eth1
sonic-cli(conf-if-Ethernet1)#

Another usacase is that, if multiple options in the same PTYPE are defined to support all possible interface options, the ext_help can be used along with ext_help attributes to provide multiple help string for corresponding options.
e.g.

<PTYPE
name="ALL_INTERFACE"
method="regexp_select"
ext_pattern="Ethernet(Port) PortChannel(1-128) Vlan(1-4093) Loopback(0..16383)"
ext_help="(Ethernet interface type) (PortChannel interface type) (Vlan interface type) (Loopback interface type)"
pattern="&ETH_ALL_INTF;|&VLAN_ALL_INTF;|&PO_ALL_INTF;|&LOOPB_INTF;"
help="Interface Type - Ranges"
/>

3. idle session timeout - CLI session timeout set to 10 minute (600s) to close the session once expired
4. Interrupt handler to abort the execution of current command
5. Allow param value with special characters, e.g. “#?\sonic&”
6. Quoted sting support , e.g. sonic-cli(conf-if-Ethernet1)# description “Azure leaf-Updating the rules file permissions that appears to be stripped #1 NY-datacenter”
7. hidden command support in klish parser
8. Patching hook log syslog message to audit command execution through CLI
9. Add Pylint support
10. Fix pipe command
11. Miscellaneous bug fixes - subcommands or param while pressing tab etc.