db/cassandra: Add support for skip_verification config

[wmrmrx]

Description

Adds skip_verification to Cassandra database plugin

Checklist

• [x] Added CHANGELOG entry (only for user-facing changes)
• [ ] Acceptance tests where run against all supported Vault Versions

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -run=TestAccXXX -timeout 30m ./...
?       github.com/hashicorp/terraform-provider-vault   [no test files]
?       github.com/hashicorp/terraform-provider-vault/cmd/coverage  [no test files]
?       github.com/hashicorp/terraform-provider-vault/cmd/generate  [no test files]
?       github.com/hashicorp/terraform-provider-vault/helper    [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/consts   [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/identity/group   [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/identity/mfa [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/pki  [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/sync [no test files]
?       github.com/hashicorp/terraform-provider-vault/schema    [no test files]
ok      github.com/hashicorp/terraform-provider-vault/codegen   0.007s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/internal/identity/entity  0.013s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/internal/provider 0.016s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/testutil  0.007s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/util  0.010s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/util/mountutil    0.004s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/vault 0.021s [no tests to run]
...

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