rafthttp: permit very large v2 snapshots

[heyitsanthony]

v2 snapshots were hitting the 512MB message decode limit, causing
sending snapshots to new members to fail for being too big.

[gyuho]

lgtm. thanks!

Will backport both to 3.1 and 3.2 before release.

[gyuho]

c.f. #5386 (comment)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@56841bb). Click here to learn what that means.
The diff coverage is 45%.

@@            Coverage Diff            @@
##             master    #8074   +/-   ##
=========================================
  Coverage          ?   76.51%           
=========================================
  Files             ?      342           
  Lines             ?    26548           
  Branches          ?        0           
=========================================
  Hits              ?    20314           
  Misses            ?     4774           
  Partials          ?     1460

Impacted Files	Coverage Δ
rafthttp/msg_codec.go	100% <100%> (ø)
integration/cluster_proxy.go	100% <100%> (ø)
rafthttp/http.go	77.1% <100%> (ø)
proxy/grpcproxy/adapter/auth_client_adapter.go	35.29% <35.29%> (ø)

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