CNS needs a IPV6 prefix cap 120 to prevent too many IPs in memory

**Component (Azure NPM or Azure CNI):**
CNS

**Describe in detail the feature/behavior/change you'd like to see:**
Introduce a config variable, make it to 120 for now, it's a safety mechanism that limits how many IPs get generated from IPv6 CIDR blocks. When an IPAssignment contains an IPv4-mapped IPv6 address (e.g. ::ffff:10.0.0.0/112) with a prefix length smaller than /120, the code narrows the prefix to /120.

**Orchestrator(e.g. Kubernetes, Docker):**
Kuberetes

**Operating System (Linux/Windows):**
Linux

**Anything else you would like to add:**
[Miscellaneous information that will assist in solving the issue.]