Feature-request : Implement Other Putmodes for HttpStore

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

`HttpStore::put_opts` only supports `PutMode::Overwrite`. Using `PutMode::Create` or `PutMode::Update` returns `Error::NotImplemented`. Every other backend (`aws`, `azure`, `gcp`) already supports both, so `HttpStore` can't be used for optimistic-concurrency writes (e.g. create-if-absent, update-if-unchanged).

**Describe the solution you'd like**

Implement `PutMode::Create` and `PutMode::Update` for `HttpStore` using standard HTTP conditional headers:

- `PutMode::Create` → `If-None-Match: *`
- `PutMode::Update(v)` → `If-Match: ""`

Map failures back to existing error types:
- `Create` + `412` → `Error::AlreadyExists`
- `Update` + `412` → `Error::Precondition`

**Describe alternatives you've considered**

Using the WebDAV `If` header (RFC 2518 §9.4) instead — not needed here since it's built for lock tokens, and `PutMode` only needs ETag-based conditions. Plain `If-Match`/`If-None-Match` is simpler and works on more generic HTTP servers.

**Additional context**

Not all servers honor conditional headers on PUT — this is optional server behavior. This change does not add WebDAV locking support.

Start at HttpStore::put_opts and compare its handling with the existing aws, azure, and gcp backends. Trace how conditional HTTP headers and 412 responses map to existing error types; done means Create and Update use the requested conditions while Overwrite remains unchanged.

Written by the indexing model from the issue text.