[Rust Server] Allow configuration of multipart/form attachment size limit

[richardwhiuk]

multipart 0.14+ imposes a 8MB size limit on multipart/form bodies - so we regressed support for > 8MB when we upgraded to multipart 0.14 a while back.

This change allows that limit to be configured at a server scope. The default is left as is, as removing the limit, or substantially increasing it, may present a Denial of Service attack risk.

This also improves error messages produced when handling multipart/form bodies.

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