Added cost monitoring data retrieval API

Description

Added cost monitoring data retrieval API with cached response

costexplorer_client is enriched with a get_cost_data method to retrieve paginated results from CostExplorer API
a new endpoint to retrieve cost data is added at /cost-monitoring/clusters/<cluster-name>
- the new API accepts two parameters, start (mandatory), end (non mandatory, defaults to now()), both are UTC ISO 8601 datetimes
right now we still need to test the actual caching by the browser (to see if the implementation on the backend is fine or not), and needed headers will be provided in a subsequent iteration (Cache-Control, Expires and Last-Modified)

[x] I added tests to new or existing code
[ ] I removed hardcoded strings and used react-i18next library (useTranslation hook and/or Trans component), see an example here
[x] I made sure no sensitive info gets logged at any time in the codebase (see here) (e.g. no user info or details, no stacktraces, etc.)
[x] I made sure that any GitHub issue solved by this PR is correctly linked
[ ] I checked that infrastructure/update_infrastructure.sh runs without any error
[x] I checked that npm run build builds without any error
[x] I checked that clusters are listed correctly
[x] I checked that a new cluster can be created (config is produced and dry run passes)
[x] I checked that login and logout work as expected

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