Closes hashicorp#12329

Vault treats all POST or PUT HTTP requests equally - they default to
being treated as UpdateOperations, but, if a backend implements an
ExistenceCheck function, CreateOperations can be separated out when the
existence check returns false.

It follows, then, that if a CreateOperation handler is implemented
without an ExistenceCheck function, this is unreachable code - a coding
error. It's a fairly minor error in the grand scheme of things, but it
causes the generated OpenAPI spec to include x-vault-createSupported for
operations on which create can never actually be invoked - and promotes
muddled understanding of the create/update feature.

In this PR:

1) Implement a new test, which checks all builtin auth methods and
   secrets engines can be successfully initialized. (This is important
   to validate the next part.)

2) Expand upon the existing coding error checks built in to
   framework.Backend, adding a check for this misuse of CreateOperation.

3) Fix up instances of improper CreateOperation within the Vault
   repository - just two, transit and mock.

Note: At this point, the newly added test will **fail**.

There are improper uses of CreateOperation in all of the following:

    vault-plugin-auth-cf
    vault-plugin-auth-kerberos
    vault-plugin-auth-kubernetes
    vault-plugin-secrets-ad
    vault-plugin-secrets-gcpkms
    vault-plugin-secrets-kubernetes
    vault-plugin-secrets-kv
    vault-plugin-secrets-openldap
    vault-plugin-secrets-terraform

each of which needs to be fixed and updated in go.mod here, before this
new check can be added.

This is a surprisingly complicated special case

…nvocations of the builtin backends too

Note, this IS an upgrade despite the apparent version numbers going
down. (That's a consequence of slightly odd release management occurring
in the plugin repositories.)

…I checks on PR can run

…dpoint

By being hidden behind a Go build constraint, it had evaded notice until
now.