[lisa] lisafs package POC.

This change mainly aims to define the semantics of communication for the LISAFS
(LInux SAndbox Filesystem) protocol. This protocol aims to replace 9P and
intends to bring some performance benefits with it.

Some of the notable differences from the p9 package are:
- Now the server implementations own the handlers.
- As a result, there is no verbose interface like `p9.File` that all servers
  need to implement. Different implementations can extend their File
  implementations to varying degrees without imposing those extensions to other
  server implementations that might not have anything to do with those features.
- If a server implementation adds a new RPC message, other implementations are
  not compelled to support it.

I wrote a benchmark `BenchmarkSendRecv` in connection_test.go which competes
with p9's `BenchmarkSendRecvChannel`. Running these on an AMD Milan machine
shows that lisafs is **45%** faster.

**With 9P**
goos: linux
goarch: amd64
pkg: gvisor/pkg/p9/p9
cpu: AMD EPYC 7B13 64-Core Processor
BenchmarkSendRecvLegacy-256     82830     14053 ns/op     633 B/op     23 allocs/op
BenchmarkSendRecvChannel-256     776971     1551 ns/op     184 B/op     6 allocs/op

**With lisafs**
goos: linux
goarch: amd64
pkg: pkg/lisafs/connection_test
cpu: AMD EPYC 7B13 64-Core Processor
BenchmarkSendRecv-256     1399610     853.5 ns/op     48 B/op     2 allocs/op

Fixes #5464

PiperOrigin-RevId: 397803163

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