Core: Expose max HTTP body size control (#218)

[Aditya-9-6]

Resolves #218 by porting PBC-Java PR prebid/prebid-cache-java#192.

Description

In high-throughput environments and large VAST store requests, operators need control over incoming HTTP body sizes to prevent unbounded memory consumption while also having the flexibility to allow legitimate larger payloads (>256KB/512KB).

This PR:

1. Adds request_limits.max_body_size_bytes configuration option in bytes (following request_limits.max_header_size_bytes convention introduced in Config max request header size #174).
2. Defaults to 262,144 bytes (256 KB), matching PBC-Java.
3. Provides backwards-compatible parsing for server.max-http-body-size-kb (PBC-Java syntax), automatically converting KB to bytes.
4. Enforces the limit in PUT requests using http.MaxBytesReader.
5. When a request exceeds the limit, returns HTTP 413 Payload Too Large (http.StatusRequestEntityTooLarge) with error message "Request body exceeded maximum size." and records bad request metrics. Setting the limit to 0 disables body size limiting.
6. Adds unit tests covering configuration validation, PBC-Java property mapping, and handler responses for within-limit, exceeded-limit, and unlimited cases.