Work through and address or remove TODOs in source for Azure Core

[ahsonkhan]

Take some action on the TODOs in source:

• Some of these TODOs are minor code refactoring or optimization investigations that can be turned into issues
• Otherwise, fix them if necessary, or file individual issues for potential features
• Else, remove them as non-actionable or out-dated

https://github.com/Azure/azure-sdk-for-c/search?l=C&q=TODO

https://github.com/Azure/azure-sdk-for-c/blob/9f5a6334863ebbf214c6e2fbd5d6e3fce52fe0de/sdk/src/azure/core/az_http_policy_retry.c#L30-L31 https://github.com/Azure/azure-sdk-for-c/blob/9f5a6334863ebbf214c6e2fbd5d6e3fce52fe0de/sdk/src/azure/core/az_json_token.c#L126-L129 https://github.com/Azure/azure-sdk-for-c/blob/36a7257760a776f8aa70517031c0b3dbc41bc5e4/sdk/src/azure/core/az_json_writer.c#L330-L331 https://github.com/Azure/azure-sdk-for-c/blob/9f5a6334863ebbf214c6e2fbd5d6e3fce52fe0de/sdk/src/azure/platform/az_curl.c#L512-L521 https://github.com/Azure/azure-sdk-for-c/blob/9f5a6334863ebbf214c6e2fbd5d6e3fce52fe0de/sdk/src/azure/core/az_span.c#L790-L791 https://github.com/Azure/azure-sdk-for-c/blob/36a7257760a776f8aa70517031c0b3dbc41bc5e4/sdk/inc/azure/core/az_json.h#L121-L139

cc @CIPop