Improve HTTP client configuration with functional options

[martinyonatann]

Problem

The generated achievement client currently uses TransportConfig for HTTP
client configuration:

client := achievementclient.NewHTTPClientWithConfig(
    strfmt.Default,
    &achievementclient.TransportConfig{
        Host:     "demo.accelbyte.io",
        BasePath: "/api/v1",
        Schemes:  []string{"https"},
    },
)

This requires callers to construct and manage a configuration struct even when only a small number of options need to be customized.

Proposed improvement

Introduce functional options for configuring the HTTP client:

client := achievementclient.NewHTTPClient(
    strfmt.Default,
    achievementclient.WithHost("demo.accelbyte.io"),
    achievementclient.WithBasePath("/api/v1"),
    achievementclient.WithSchemes([]string{"https"}),
)

Proposed options:

• WithHost
• WithBasePath
• WithSchemes
• WithUserAgent