Support custom Content-Type header

[hatamiarash7]

This pull request enhances the configuration and usability of the http-echo Go web server. The main improvements include support for setting the HTTP response Content-Type header via a command-line flag, updates to the README for clearer documentation, and adjustments to handler functions to accommodate the new configuration option.

Documentation improvements:

• Expanded the README.md to include a configuration table detailing all command-line arguments, their defaults, and descriptions, making it easier for users to understand available options.

Feature additions:

• Added the -content-type command-line flag in main.go, allowing users to customize the Content-Type header of HTTP responses. The returned content-type header for all responses is text/plain; charset=utf-8 right now.
• Updated the withAppHeaders function in handlers.go to accept and set the Content-Type header in responses, supporting the new flag.

Integration and usage:

• Modified the main handler registrations in main.go to pass the Content-Type value from the flag to all endpoints, ensuring consistent header behavior.

[hashicorp-cla-app]

All committers have signed the CLA.