Add functionality to parse multiple prefixes

[mproffitt]

This change introduces a new function multiprefixloop as a convenience function for splitting multiple cidr prefixes govorned by the number of bits and required offset inside each prefix.

This function might be used in instances such as dividing additional VPC cidrs into subnets without requiring multiple instances of the funtion to be declared in a given composition.

Additionally this PR:

• Contributes towards issue #2 by introducing happy path unit tests for each function.
• Offers additional documentation for the function input definition
• Introduces the missing make debug command referenced in the readme

Description of your changes

Fixes #

I have:

• [x] Read and followed Upbound's contribution process.
• [ ] ~Run make reviewable to ensure this PR is ready for review.~
• [ ] ~Added backport release-x.y labels to auto-backport this PR, as appropriate.~

How has this code been tested

By introducing tests that contribute towards ensuring stability of the function

go test -timeout 30s -run ^TestRunFunction$ github.com/upbound/function-cidr -v -count=1 -timeout=30s -parallel=4 -cover
=== RUN   TestRunFunction
=== RUN   TestRunFunction/cidr-host
=== RUN   TestRunFunction/cidr-subnet
=== RUN   TestRunFunction/cidr-netmask
=== RUN   TestRunFunction/cidr-subnets
=== RUN   TestRunFunction/cidr-subnetloop
=== RUN   TestRunFunction/multi-prefix-loop
--- PASS: TestRunFunction (0.00s)
    --- PASS: TestRunFunction/cidr-host (0.00s)
    --- PASS: TestRunFunction/cidr-subnet (0.00s)
    --- PASS: TestRunFunction/cidr-netmask (0.00s)
    --- PASS: TestRunFunction/cidr-subnets (0.00s)
    --- PASS: TestRunFunction/cidr-subnetloop (0.00s)
    --- PASS: TestRunFunction/multi-prefix-loop (0.00s)
PASS
coverage: 52.4% of statements
ok      github.com/upbound/function-cidr        0.015s  coverage: 52.4% of statements

Additionally by validating that there is no change to execution through crossplane beta render using make debug and make render commands