Turn on task persistence by default

[desertaxle]

To ensure we get idempotency for tasks out of the box, we need to ensure that tasks persist their results by default. With this PR, persist_result will default to True for all tasks. If persist_result is set to false, then the cache_policy will be ignored.

To facilitate writing results by default, we will attempt to load a default storage block matching the default storage block configured with PREFECT_DEFAULT_RESULT_STORAGE_BLOCK. If that block doesn't exist (defaults to {hostname}-storage), we will create and save that block for the user. That block will be reused for subsequent runs.

Closes #14087

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[cicdw]

One request on the init logic, otherwise LGTM!

[cicdw]

Good stuff, LGTM

[desertaxle]

Related to #13800