Add decode_access_token to client

[Aetylus]

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What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [x] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

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Resolves #764

Notes:

• Currently only implements for the async Starlette client, but if implementation looks good, I can implement the sync version as well.
• Initial tests have some duplicated code, but I was hesitant to start adding utility functions just for this to avoid that.