feat: add merge support for casc defined system credentials

[cronik]

Enables support for merging casc defined credentials with existing credentials (i.e. manually created).

In some environments it is desirable to define some credentials declaratively while also be able to define credentials through the UI (which out otherwise be defined in plain text or in encoded format which could be easily transferable).

One such example is mixing vault credentials defined in casc config with those defined directly through Jenkins. In the below casc example it is assumed the the vault-approle credential was created through the Jenkins UI. Currently this is not possible since the casc system credentials will remove all non-casc defined credentials on restart.

unclassified:
  hashicorpVault:
    configuration:
      vaultCredentialId: "vault-approle"
      vaultUrl: "https://myvault.mycorp.net"
credentials:
  system:
    domainCredentials:
      - credentials:
          - vaultUsernamePasswordCredentialImpl:
              description: "vault managed credential"
              id: "my-vault-cred"
              path: "kv/jenkins"
              scope: GLOBAL

To enable merging behavior set the env var CASC_CREDENTIALS_MERGE_STRATEGY=merge or system property -Dcasc.credentials.merge.strategy=merge

fixes JENKINS-64079

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