Fix/init blocking

[elizabethengelman]

What

• moves the init integration test into the integration dir
• makes the init command async, and then uses the async reqwest client.

Why

I was seeing this error when running contract init on main:

thread 'main' panicked at .cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.39.2/src/runtime/blocking/shutdown.rs:51:21:
Cannot drop a runtime in a context where blocking is not allowed. This happens when a runtime is dropped from within an asynchronous context.
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This happens because we were trying to use the blocking reqwest client inside of an async runtime, since all of the cli is in an tokio runtime. This doesn't work well together, so I've changed the init command to be async and then we're able to await the HTTP calls.

Known limitations

This won't be an issue once https://github.com/stellar/stellar-cli/pull/1628/files is merged - but we'll still want to move the init test into the integration dir - I think that this test was previously not being run.

[willemneal]

Good catch

[leighmcculloch]

Because #1628 is merged, does anything from this change still need to be merged?