feat: add --anvil-header flag for customizing HTTP responses

[crStiv]

Motivation

closes #8425.

Solution

• Added new --anvil-header CLI flag that accepts multiple header values
• Implemented header parsing and storage in ServerConfig
• Modified HTTP response handler to include these headers in all responses
• Used the same format as the existing --fork-header flag for consistency ("Name: Value")

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[mattsse]

while this isn't super invasive and isolated to the HTTP handler, I'm not sure we really need this

[crStiv]

while this isn't super invasive and isolated to the HTTP handler, I'm not sure we really need this

oh... I've made changes and only after implementing them I've noticed what you've written

yeah I understand that it might not be what you need rn. but I'm just trying to do an issue, so what do you think I have to do rn?

is there any sense to continue working on this issue or nah?