allow JSON values in Secret block

[zzstoatzz]

this PR closes #14899 by allowing any JSON value (i.e. strings, lists, dictionaries) to be stored as a secret

examples

In [2]: Secret.load('test').get()
Out[2]: [1, 2, 3]

In [3]: type(_)
Out[3]: list

from pydantic import Secret as PydanticSecret

from prefect.blocks.system import Secret

Secret(value={"key": "value"}).save("secret-block", overwrite=True)

secret_block = Secret.load("secret-block")

assert secret_block.get() == {"key": "value"}
assert secret_block.value == PydanticSecret[dict]({"key": "value"})

---

this PR should not change any behavior for current users of Secret, including this outstanding issue about stripping newlines

[codspeed-hq]

CodSpeed Performance Report

Merging #14980 will not alter performance

_{Comparing generic-secret-block (bcb83fa) with main (491af65)}

Summary

✅ 4 untouched benchmarks

[cicdw]

As a manual sanity check, I want to test a load against an "old" secret stored somewhere but otherwise this LGTM

[zzstoatzz]

As a manual sanity check, I want to test a load against an "old" secret stored somewhere but otherwise this LGTM

coming back to this, I've checked this against all the old secrets I've had defined before - have you found any cases of unexpected behavior?

[desertaxle]

Two nits on docstring changes, but otherwise LGTM!