Fix: update PTB client method for fetching coins

[faisal-chainlink]

This PR updates the PTB client to ensure the following points:

1. Getting all the coins owned by an account makes multiple RPC calls to get around the 50 coins response limit. This solves the edge case of having too many coins.
2. A new method to query for a specific coin type rather than getting all of the owned coins by the account.

[github-actions]

👋 faisal-chainlink, thanks for creating this pull request!

To help reviewers, please consider creating future PRs as drafts first. This allows you to self-review and make any final changes before notifying the team.

Once you're ready, you can mark it as "Ready for review" to request feedback. Thanks!