Add google_compute_instance to v6 cai2hcl

[zli82016]

1. The cai2hcl conversion workflow for google_compute_instance is cai asset -> API object -> tfplan. In this way, the flatten functions in the provider, which convert API object into Terraform resource, can be reused. To handle the inconsistency between cai asset and API object, some modification is needed for the flatten functions.
2. The fields with default value in provider are only converted when their values are not default.
3. Testing:

In magic modules, generate tgc

make tgc  OUTPUT_PATH="$GOPATH/src/github.com/GoogleCloudPlatform/terraform-google-conversion"

In terraform-google-conversion

make build
./bin/tgc cai2hcl convert full_compute_instance.json > full_compute_instance.tf

The automatic testing will come soon.

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

terraform-google-conversion: Diff ( 4 files changed, 561 insertions(+), 4 deletions(-))