[Deployments]: New builder service

[jonaro00]

Shuttle's new builder service is a major milestone for Shuttle that unblocks many other features.

These are the completed features so far:

• Faster builds
• Choose Rust toolchain version, including nightly (Allow specifying rust toolchain/version #1125)
• (partial support: https://docs.shuttle.dev/docs/builds#environment-variables) Build-time environment varaibles
• Build and run integration tests with Shuttle resources
• Add external build and runtime dependencies ([Feature]: Add external dependencies to project #703)
• (possible with hook scripts) Support building WASM frontends with Trunk
• (possible with hook scripts) Support building JS frontend assets
• Customize the build command (enables fullstack Leptos with SSR) ([Bug]: Leptos SSR feature doesn't work nicely with shuttle #1002)
• Custom linker (Allow using a different linker in deployment #1000)

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