Feature-request : Implement Other Putmodes for HttpStore

[giridher-art]

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: "<v.e_tag>"

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.