Add a way to setup raw builds for all channels

[annagrin]

This allows developers relying on SDK changes to land and test changes
in other packages using a raw dev build so we can validate changes,
publish them, and roll to dependent packages (such as flutter) faster.

Closes: #29

[sigmundch]

Looks good to me, in my attempt (#37) I used a "signed" boolean instead of the raw/release strings as a parameter. My rationale was to hide the name of the folders and make it more about the semantics (one is signed the other is unsigned and unpublished).

But honestly I was torn between both options. Maybe @athomas has a preference?

I also explored an option of using a dev_unpublished channel, but I think the flavor/signed option is better.

[annagrin]

@sigmundch I am fine either way, I'll wait for @athomas to comment.

[sigmundch]

@whesse would you be able to take a look while @athomas is on vacation?

[mit-mit]

Thanks for the update, LGTM for merging

[whesse]

I just tested downloading the ARM64 MacOS raw build of the SDK, and running dart on a Mac M1 machine, and it worked. So it will be fine to use the unsigned raw bits, I believe, even on the one configuration which might require a signed one. (Actually, the raw build is signed too, just self-signed with a local certificate).

[annagrin]

@whesse thank you for testing this! I made this change to make iteration over cross-repo features faster (for example, webdev can test against raw dev SDK bits in its CI which are available faster than the release bits), hopefully it can be helpful for others as well.