add --http-headers option.

[fujiwara]

This pull request introduces several changes to enhance the HTTP configuration and handling in the stretcher package. The main updates include adding support for custom HTTP headers and passing configuration options to various functions.

Enhancements to HTTP configuration and handling:

• http.go: Modified the HTTPOptions struct to include a help description for the Headers field.

• stretcher.go: Added the HTTPOptions struct to the Config struct to embed HTTP configuration options.

Updates to function signatures to include configuration options:

• stretcher.go: Updated the getHTTP function to accept an HTTPOptions parameter and added logic to include custom headers in HTTP requests. [1] [2]

• stretcher.go: Modified the getURL function to accept a Config parameter and updated its calls to pass the configuration.

• manifest.go: Updated the fetchSrc function to pass the Config parameter to the getURL function.

• stretcher.go: Updated the getManifest function to accept a Config parameter and pass it to the getURL function. [1] [2]