HTTP zstd accept-encoding window size

The Zstandard RFC (8478 <https://www.rfc-editor.org/rfc/rfc8478>) defines
the compression algorithm as well as the content-encoding that is used for
HTTP.

The compression algorithm itself allows for backreference window sizes up
to 2 GB but there is language recommending decoders support at least up to
8 MB and encoders not generate frames requiring a window size larger than 8
MB:

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   In order to protect decoders from unreasonable memory requirements, a
   decoder is allowed to reject a compressed frame that requests a
   memory size beyond decoder's authorized range.

   For improved interoperability, it's recommended for decoders to
   support values of Window_Size up to 8 MB and for encoders not to
   generate frames requiring a Window_Size larger than 8 MB.  It's
   merely a recommendation though, and decoders are free to support
   larger or lower limits, depending on local limitations.

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There are a lot of use cases for Zstandard outside of http that justify the
need for large window sizes but the "zstd" content encoding registration
specific to http doesn't specify a limit.

I'm not sure if it belongs here or in a different wg to update, but for
compatibility of http clients and servers can we update the "zstd"
content-encoding registration to specify a maximum window size as part of
the registration?

Chrome 117 has zstd support (behind a flag) with a decoder limit of 8 MB
based on the recommendations (and to prevent abuse/attack) but we'd feel a
lot more comfortable if the agreed-upon limit was explicit.