Add generic handler for requests in APIBaseTask

[bjtitus]

• Unifies request + response handling for APIBaseTask instead of having individual methods for get, post, and delete.
• Adds unit tests to verify behavior

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Checklist

• I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• I have considered adding unit tests for my changes.
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

[dangermattic]

	2 Warnings
⚠️	PR is not assigned to a milestone.
⚠️	No reviewers have been set for this PR yet. Please request a review from @ Automattic/pocket-casts-ios.

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