[build][fuzzing] Allow fuzzers to customize sandbox

fxrev.dev/428892 removed the unused `cmx` property from the `fuzzer()`
template, but in some cases a fuzzer still needs to request specific
features and services in its sandbox. This CL addresses that by adding
back narrowly-scoped `features` and `services` properties to the
template.

Additionally, an `example-fuzzers/extended_sandbox` fuzzer is added as a
basic test for this feature.

Test: fx test extended_sandbox_test
Change-Id: Ia594e26390cc6b0abf96813c670dbf20dd67f268
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/450935
Reviewed-by: Aaron Green <aarongreen@google.com>
Testability-Review: Aaron Green <aarongreen@google.com>
Commit-Queue: Cameron Finucane <eep@google.com>