internal/http3: limit the size of decoded QPACK field names and values

[GOVINSAGA]

readPrefixedStringWithByte allocates a buffer for a field name or value
based on the length encoded on the wire, before reading the string data.

The existing check bounds that length by the read limit of the current
frame, but a frame length is itself an unbounded QUIC varint, so a peer
can declare an arbitrarily large field section in a few bytes and cause
a correspondingly large allocation without sending any string data.

Reject strings longer than 1 MiB, as HPACK does for HTTP/2.

Fixes golang/go#78702

[gopherbot]

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