plasma: add testnet compose + assets (reth + plasma-consensus)

Canonicalize plasma-testnet so the tracked compose adopts the same
volume/service names as the de-32 hand-written override (rpc_plasma-
testnet-reth-pruned-trace-{execution,consensus,jwt}). Also refresh
mainnet observer sed to be chain-agnostic.

plasma/reth/plasma-mainnet-reth-pruned-trace.yml

@@ -207,11 +207,12 @@ services:
     entrypoint: [/bin/bash, -c]
     command:
       - |
-        sed 's|http://plasma-mainnet:8551|http://plasma-mainnet:8551|g' /config/non-validator.toml > /tmp/non-validator.toml
+        # Rewrite engine_api_url to this compose service (mainnet/testnet tomls ship chain-specific hosts).
+        sed -E 's|http://[^[:space:]/"]+:8551|http://plasma-mainnet:8551|g' /config/non-validator.toml > /tmp/non-validator.toml
         exec plasma-cli observer \
           --config-path /tmp/non-validator.toml \
           --no-color \
-          --timeout-stream-manager 10ms \
+          --timeout-stream-manager 5s \
           --telemetry.enable-metrics \
           --telemetry.enable-health \
           --telemetry.host 0.0.0.0 \