feat: Copy xrc_mock from tvl in ic repo

[dskloetd]

Motivation

It can be useful to have an exchange rate canister available in a test environment without it making API calls to exchanges.
Instead of everybody implementing their own mock, we should provide one in this repo so it's easy to find and share.

In plan to use this in the snsdemo repo which creates the test environment used by nns-dapp.

Changes

1. Copied https://github.com/dfinity/ic/tree/master/rs/rosetta-api/tvl/xrc_mock to src/xrc_mock
2. Slightly changed the Cargo.toml to work in the current repo.
3. Removed BUILD.bazel as this repo doesn't use bazel.
4. Added a build script.
5. Fix the docker build workflow via cargo-culting.

I plan to include a built wasm in the release artifacts in the future PR.

Tests

I don't think the existing mock that was copied has unit tests, but I used the mock built from this repo in my test environment to test the tvl replacement in nns-dapp.