[Aukevanoost]

Part of #188 (points 1 and 2). #188's point 4 was already fixed on main for #166 (d3da91e, 939a54e) and needs no change here.

• Plugins reach esbuild. runRemoteBuilder now passes plugins to the adapter, the same way build does (src/builders/build/builder.ts). Before this, a remote dropped every custom plugin it would build with under build. Both consumers, the exposes/mappings bundle and the shared externals, already read builderOptions.plugins. NfRemoteInternalOptions.plugins is now optional, because createBuilder never supplies it.
• runRemoteBuilder is exported from @angular-architects/native-federation/internal. A custom builder can then wrap remote to add plugins, as it already can for build. Importing /internal still runs build's env setup before remote's. The env vars it sets are the same ones remote's own setup would set, so remote's replay of them logs nothing.

The remote builder is experimental, so the exported signature carries no SemVer promise.

Testing

• pnpm typecheck, pnpm test (457 passing) and pnpm lint pass.
• No new spec. The change is a one-line passthrough, like build's, and shared-bundle-settings.spec already covers that plugins get used.