[ Service ] Add HTTP and DevFS support to package:dart_runtime_service

This change adds support for invoking RPCs via HTTP requests, allowing
for interacting with the service without needing to establish a web
socket connection.

This change also adds support for the development file system, otherwise
known as DevFS. DevFS is a (currently) undocumented feature provided by
the VM service that gives clients limited file system access within a
directory contained in the system's temp directory. This is currently
used by Flutter to push kernel files to the device when performing a hot
reload.

The DevFS implementation for package:dart_runtime_service_vm removes
the long deprecated support for `path` parameters, leaving `uri`s as
the only supported format for specifying file system types. Existing
DevFS tests have been updated to replace `path` with `uri` in
preparation for dart_runtime_service_vm becoming the new default VM
service.

This change brings the package:vm_service test suite pass rate to ~95%.

Change-Id: Ib7d95db5788c37408d3dec79926ca7206660a43f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/488280
Reviewed-by: Jessy Yameogo <yjessy@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>