[optimizer] Add static and runtime mode

[sebastianbenz]

AMP Optimizer usually runs either at build time or at runtime. Both modes have very different constraints in regards to execution time:

1. Runtime: AMP Optimizer is executed in the critical rendering path requiring minimal execution times.
2. Build time: AMP Optimizer is used at build time (e.g. for static sites). In this case, execution time is less critical enabling more expensive optimizations.

Steps we should take to better support this:

1. Provide two different out-of-the box configurations for runtime and build time usage (see Add minimal Optimizer Transformer Config #1180 for a start). The runtime config should only perform AMP specific server-side optimizations, but avoid any expensive optimizations such as minification.
2. Improve default install times by making all non-essential dependencies (cssnano-simple, postcss, terser) optional.