Add support for PATCH operations

[averche]

Describe the feature request or question

We should add support for PATCH operations, which are currently missing from the two generated libraries and the underlying openapi.json.

Additional information

As far as I know, PATCH is currently used by KvV2 and a couple PKI endpoints (1, 2). We do not currently account for it in openapi.go.

We could also consider adding something like JSONMergePatch, but if the problem can be solved at the OpenAPI level, I don't think it's necessary.

[maxb]

This ties in with #195 as using PATCH introduces new ways in which omitting a field, vs. including it set to a zero value, has meaning.

Technically JSON Merge Patch also assigns meaning to including a field explicitly set to null, though it's an open question whether this would ever be useful in the context of Vault configuration endpoints.