Use asset type enum string for trustline tables

[amishas157]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with the jira ticket associated with the PR.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated the docs and README with the added features, breaking changes, new instructions on how to use the repository.

Release planning

• I've decided if this PR requires a new major/minor/patch version accordingly to
  semver, and I've changed the name of the BRANCH to release/* , feature/* or patch/* .

What

This PR updates the values of AssetType for Trustlines tables from integer to string. Related PR in upstream pipeline stellar/stellar-etl#268

Why

This is done to make AssetType consistent across all the schemas as it was mistakenly added as integer earlier.

Known limitations

None