feat(add): add support for naming pinned CIDs (#10877)

* feat(add): add support for naming pinned CID

Signed-off-by: kapil <kapilsareen584@gmail.com>

* fix(add): no double pinning and simplify pin-name

- modify PinRoot to accept name parameter, eliminating double pinning
- remove automatic filename fallback logic for cleaner behavior
- only create named pins when explicitly requested via --pin-name=value
- replace NoPinName constant with idiomatic empty string literals
- Update help text and tests to reflect explicit-only behavior

* docs: changelog

* chore: lint

* test: negative case for empty pin-name

* chore: gofmt

---------

Signed-off-by: kapil <kapilsareen584@gmail.com>
Co-authored-by: Marcin Rataj <lidel@lidel.org>

core/coreapi/test/path_test.go

@@ -39,7 +39,7 @@ func TestPathUnixFSHAMTPartial(t *testing.T) {
 		dir[strconv.Itoa(i)] = files.NewBytesFile([]byte(strconv.Itoa(i)))
 	}
 
-	r, err := a.Unixfs().Add(ctx, files.NewMapDirectory(dir), options.Unixfs.Pin(false))
+	r, err := a.Unixfs().Add(ctx, files.NewMapDirectory(dir), options.Unixfs.Pin(false, ""))
 	if err != nil {
 		t.Fatal(err)
 	}