Refactor ibm_database resource and datasource to introduce backend abstraction

[obai-1]

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Relates #0000

Summary

This PR introduces backend abstraction for both the ibm_database resource and datasource to separate Classic and Gen2 implementation paths. This is a preparatory refactor to enable incremental addition of Gen2 support without changing current behavior.

Details

• Adds a backend interface and concrete backend structs for Classic and Gen2 paths.
• Routes existing Classic resource and datasource logic through the new backend layer.
• Introduces Gen2 backend scaffolding only (no functional implementation yet).
• Gen2 plans will continue to behave exactly as before (currently erroring).
• No schema, API, or user-visible functional changes.

This is the first step in a planned series of incremental changes to support Gen2 databases while keeping risk low and avoiding large one-time refactors.

Testing

No functional changes expected. Existing acceptance tests pass.

Output from acceptance testing:

$ make build
$ make testacc TEST=./ibm/service/database TESTARGS='-run=TestAccIBMDatabaseInstance_Redis_Basic'
=== RUN   TestAccIBMDatabaseInstance_Redis_Basic
=== PAUSE TestAccIBMDatabaseInstance_Redis_Basic
=== CONT  TestAccIBMDatabaseInstance_Redis_Basic
--- PASS: TestAccIBMDatabaseInstance_Redis_Basic (633.08s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/database	634.608s

$ make testacc TEST=./ibm/service/database TESTARGS='-run=TestAccIBMDatabaseDataSourceBasic'
=== RUN   TestAccIBMDatabaseDataSourceBasic
=== PAUSE TestAccIBMDatabaseDataSourceBasic
=== CONT  TestAccIBMDatabaseDataSourceBasic
--- PASS: TestAccIBMDatabaseDataSourceBasic (529.11s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/database	530.638s
...

Output form unit testing:

$ make test TEST=./ibm/service/database
==> Checking that code complies with gofmt requirements...
go test -i ./ibm/service/database || exit 1
?   	github.com/IBM-Cloud/terraform-provider-ibm	[no test files]
echo ./ibm/service/database | \
		xargs -t -n4 go test  -timeout=30s -parallel=4
go test -timeout=30s -parallel=4 ./ibm/service/database
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/database	1.646s

[pushpalathaeg]

LGTM