Add support for /image api

[strzibny]

Documentation: https://serpapi.com/image-api

[ilyazub]

Opus 5.5 Max wrote this:
#17 is merged (7acfb80); against main this PR now conflicts in 4 files (README.md, pkg/api/client.go, pkg/api/client_test.go, pkg/cmd/search.go). What image joins on rebase: errors print as one JSON line on stderr and exit 2 when the command is wrong, which clierrors here already gives; commands build their client with newClient (pkg/cmd/root.go), which applies --timeout and SERPAPI_TIMEOUT; and stdin comes through cmd.InOrStdin(), which the in-process tests in pkg/cmd drive.