object: add target-side CSV select MVP

[Rohithmatham12]

Summary

• Add ais object select as a limited CSV MVP that routes through AIS instead of filtering locally in the CLI.
• Add a select-obj object POST action; the proxy redirects to the owning target, and the target reads the source object via LOM.GetROC and streams selected CSV rows.
• Add api.SelectObjectReader for callers that want the server-side object-select response stream.
• Keep the first scope narrow: CSV input/output, SELECT projection, SELECT *, optional FROM, and simple string/numeric WHERE predicates.
• Add shared selector coverage plus a target-side regression that stores a CSV object and exercises the target selection path.

Why
This replaces the closed client-side prototype in #310 with the server/target-side MVP requested in review. The CLI no longer performs a normal full-object GET and filters locally; it POSTs an object action to AIS, and the target performs the read/filter/stream path.

Scope / non-goals

• No S3 Select compatibility yet.
• No Parquet/JSON/DuckDB integration yet.
• No predicate pushdown into remote backend format readers yet.
• This is a stage-one CSV path intended to make the API, proxy routing, target execution, and streaming response shape reviewable.

Testing

• GOCACHE=/private/tmp/aistore-select-gocache GOMODCACHE=/private/tmp/aistore-select-gomodcache go test ./cmn
• GOCACHE=/private/tmp/aistore-select-gocache GOMODCACHE=/private/tmp/aistore-select-gomodcache go test ./api
• GOCACHE=/private/tmp/aistore-select-gocache GOMODCACHE=/private/tmp/aistore-select-gomodcache go test ./ais -run TestObjSelectRunsOnTarget
• cd cmd/cli && GOCACHE=/private/tmp/aistore-select-gocache GOMODCACHE=/private/tmp/aistore-select-gomodcache go test ./cli
• git diff --check

Replaces the closed client-side prototype #310.
Refs #305.

[alex-aizman]

1. unacceptable heap allocation in the critical path
2. matchSelectPredicate is called in the main loop (for each row) and parses the same user-given value - unacceptable as well

[alex-aizman]

Closing (comment inline)