Unify the direct Compute API, Mapper and Fuzzer

[yuwenma]

The Compute direct controllers are structured in its own directory, which confuses the mapper-generator tool that places all auto-generated code in a single mapper.generated.go file.

This blocks us migrating and promoting other Compute resources.

Actions

• Add generate.sh to ./apis/compute/v1beta1
• Flatten the subdirectories FirewallPolicyRule, ForwardingRule and TargetTCPProxy.
• Fix the 3 resources mapper functions
• Fix the 3 resources fuzzer functions

[gemmahou]

@yuwenma This has been addressed in the 4 linked PRs(They are expected to be reviewed and merged sequentially, as each one builds on the previous one.). Any comments are appreciated.