Move DTOne airtime service implementation into this repo

[rowanseymour]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.54839% with 10 lines in your changes missing coverage. Please review.

Project coverage is 53.73%. Comparing base (3e35dd0) to head (1ba85a7).

Files with missing lines	Patch %	Lines
services/airtime/dtone/client.go	92.13%	5 Missing and 2 partials ⚠️
services/airtime/dtone/service.go	95.45%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #545      +/-   ##
==========================================
+ Coverage   53.30%   53.73%   +0.43%     
==========================================
  Files         209      211       +2     
  Lines       14307    14462     +155     
==========================================
+ Hits         7626     7771     +145     
- Misses       5860     5867       +7     
- Partials      821      824       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.