RELIC: Make Typed Headers Extensible for Custom Implementations

[klkucaj]

Is your feature request related to a problem? Please describe.
The framework currently provides predefined typed headers (e.g., AcceptEncodingHeader) for known HTTP headers. However, developers cannot extend or override these typed headers to provide their own implementations, limiting flexibility for certain use cases.

Describe the solution you'd like
Allow developers to implement or override typed headers by:

• Making typed headers abstract by default.
• Providing default implementations for common headers (e.g., AcceptEncodingHeader).
• Ensuring the framework supports custom implementations seamlessly.

Describe alternatives you've considered

• Continue relying solely on framework-provided typed headers, which simplifies the API but restricts extensibility.
• Introduce a mechanism to dynamically register or replace typed headers at runtime, which adds complexity.

Additional context
This request is specific to typed headers (known HTTP headers with structure and validation) and does not relate to arbitrary custom headers like Company-ID. Allowing extensibility ensures the framework remains flexible without compromising its default functionality.

Feedback: serverpod/serverpod#2744 (comment)