…Liquidity Pool trades (#685)

### What

Add GitHub Actions to run tests for every PR.

### Why

So all AMM related PRs we're merging into [v9.0.0](https://github.com/stellar/js-stellar-sdk/tree/v9.0.0) will need to pass the CI tests.

…ounts balance response (#688)

Support `/accounts?liquidity_pool={:pool_id}` and update the response from `/accounts/{:acc_id}`

### Update

- A new type of trustline called `liquidity_pool_shares` was added, which is included in the account `balances` array and causes a breaking change (see below) ([#688](#688)).
- Clients can now filter the results from the `/accounts` endpoint based on participation in a certain liquidity pool ([#688](#688)).

### Breaking Changes

- The `balances` array from an account response now supports liquidity pool balances ([#688](#688)).
  - the `asset_type` field can now be `liquidity_pool_shares`.
  - `buying_liabilities`, `selling_liabilities`, `asset_code`, and `asset_issuer` are omitted from the response for pool shares because they are not relevant to liquidity pools.
- Update the `ChangeTrustOperationResponse` interface so it can conform to a change in a liquidity pool trustline ([#688](#688)).
  - `asset_type` can now be `liquidity_pool_shares`.
  - `asset_code` and `asset_issuer` are now optional.
  - Added `liquidity_pool_id` as an optional field.