IC-128: First iteration of serving assets

[dsarlis]

This adds a very first implementation of serving assets from the IdP canister. I'm using two hard-coded assets for now to be able to implement the basic pieces of the interface. It basically initializes a map of assets during canister_init and then exposes the http_request query function required for the HTTP gateway to ask for assets.

Work left for future PRs:

• This does not include the StreamingStrategy yet in the interface (see here for what's a StreamingStrategy), I need to figure out how to do that in Rust in a way that Candid understands.
• Assets should be dynamically set during canister_init, likely as an input argument.

Testing

To install the canister locally:

cd idp_service/src
dfx start --background
dfx canister create idp_service
dfx build idp_service
dfx canister install idp_service

Then you can ask for an asset:

dfx canister call idp_service http_request '(record {method="GET"; url="/sample-asset.txt"; headers=vec {}; body=vec{};})'
(
  record {
    1_092_319_906 = blob "This is a sample asset!\0a";
    1_661_489_734 = vec {};
    3_475_804_314 = 200;
  },
)

If an asset does not exist:

dfx canister call idp_service http_request '(record {method="GET"; url="/where-is-my-asset.txt"; headers=vec {}; body=vec{};})'
(
  record {
    1_092_319_906 = blob "Asset /where-is-my-asset.txt not found.";
    1_661_489_734 = vec {};
    3_475_804_314 = 404;
  },
)