Add Beacon.Plug to `mix beacon.install` #341

Job	Run time
quality: OTP 27 \| Elixir 1.17 \| Phoenix ~> 1.7 \| LV ~> 1.0.0-rc.7	46s
test: OTP 23 \| Elixir 1.14.0 \| Phoenix 1.7.0 \| LV 0.20.2 \| PG 13.0-alpine	4m 55s
test: OTP 27 \| Elixir 1.17 \| Phoenix ~> 1.7 \| LV ~> 1.0.0-rc.7 \| PG 16.4-alpine	4m 20s
	10m 1s

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