HPR-1170: Add hcp_packer_bucket_names data source.

[aidan-mundy]

🛠️ Description

• Adds hcp_packer_bucket_names data source.
• Adds documentation for more automated management of channels.

🏗️ Acceptance tests

• Have you added an acceptance test for the functionality being added?
• Have you run the acceptance tests on this branch?

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAcc.*PackerBucket.* -test.v'
==> Checking that code complies with gofmt requirements...
golangci-lint run --config ./golangci-config.yml 
TF_ACC=1 go test ./internal/... -v -run=TestAcc.*PackerBucket.* -test.v -timeout 360m -parallel=10
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/clients    (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/consul     (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/input      (cached) [no tests to run]
=== RUN   TestAcc_dataSourcePackerBucketNames
--- PASS: TestAcc_dataSourcePackerBucketNames (21.94s)
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/provider   22.467s

[sylviamoss]

👍🏼 LGTM!