pesapal: Implement get, post, and request functions

[hunterachieng]

Summary

Implement get, post, and request functions for the new adaptor to create transactions, get transactions status.

Fixes #993

Details

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Review Checklist

Before merging, the reviewer should check the following items:

• Does the PR do what it claims to do?
• If this is a new adaptor, added the adaptor on marketing website ?
• If this PR includes breaking changes, do we need to update any jobs in
  production? Is it safe to release?
• Are there any unit tests?
• Is there a changeset associated with this PR? Should there be? Note that
  dev only changes don't need a changeset.
• Have you ticked a box under AI Usage?