Support all backend configuration option defaults for dynamic backends

[guybedford]

Currently the enableDynamicBackends call allows providing default configuration options for dynamic backends.

These defaults are restricted to only the three timeout values currently.

It would be beneficial to extend these defaults to all configuration options that make sense across all backend constructor calls generally.

Another way to frame this might even be as default construction options for all backends possibly even as fallbacks for options not provided to new Backend(). Under such a framing it might become import { setDefaultBackendOptions } from 'fastly:backend' say.

This would align well with eventually deprecating enableDynamicBackends() entirely, which could nice to get going for the next major release, to rather give a nicer error when dynamic backends are disabled.