Add Beacon.Plug to `mix beacon.install` (#686) #351

Job	Run time
test: OTP 23 \| Elixir 1.14.0 \| Phoenix 1.7.0 \| LV 0.20.2 \| PG 13.0-alpine	2m 59s
test: OTP 27 \| Elixir 1.17 \| Phoenix ~> 1.7 \| LV ~> 1.0.0-rc.7 \| PG 16.4-alpine	2m 4s
quality: OTP 27 \| Elixir 1.17 \| Phoenix ~> 1.7 \| LV ~> 1.0.0-rc.7	2m 9s
	7m 12s

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