Kms submodules

[chris-giblin]

This PR can be viewed as a study but also as a real candidate for main. It incorporates:

• suggestions made during PR Vault #30
• a refactoring of kms into submodules (issue Design KMS plugin strategy #32).
• a new kms provider for IBM Cloud Key Protect (issue Implement kms provider for IBM Cloud Key Protect #34)

The kms refactoring replaces the type field in the config json with a field for the classname of the respective kms provider. This allows easy drop-in of new kms providers, not requiring source changes to KmsFactory. For example,

[
  {
    "name"         : "vault-test",
    "kmsClassname" : "io.strimzi.kafka.topicenc.kms.vault.VaultKms",
    "uri"          : "http://127.0.0.1:8200/v1/secret/data/test",
    "credential"   : "s.3gxh8p83kxkTv9IB66GmORtK"
  },
  {
    "name"         : "test",
    "kmsClassname" : "io.strimzi.kafka.topicenc.kms.test.TestKms"
  },
  {
    "name"         : "cloud",
    "kmsClassname" : "io.strimzi.kafka.topicenc.kms.keyprotect.KeyProtectKms",
    "uri"          : "http://sfsffs.ibm.com/abc",
    "instanceId"   : "shdjkjahkjahd",
    "credential"   : "<apikey>"
  }
]

[tombentley]

Thanks @chris-giblin. I took a quick look and this looks promising. I think it needs rebasing before I can give it a proper review, because it includes a lot of stuff from the recently reviewed/merged PRs so it's difficult to see the wood for the trees right now.

[chris-giblin]

Agree, things have become messy. I am closing this PR and will work up to it in a series of smaller steps.