Fix handling of very large numbers in json. (#864)

The fix was very easy: enable `arbitrary_precision` in JSON.

The issue was reported by a customer and manifested in a very annoying
and obscure manner.

Essentially what's going on: massive numbers in the schema (e.g.
-1.7976931348623157e+308) were being handled correctly when passed this
way, but not when passed as the full number with all of the 308 zeros
after it. This won't be a problem usually, as these numbers are wacky,
but there was a bug that was triggered with event type schemas. As
mentioned, we accept the `e+308` form, but rejected the full number with
all the zeros. The problem is that when saving schemas to postgres we
are saving it to a jsonb field which doesn't save the original json
structure. The field was saved without a problem (because of the short
representation), but once read from postgres, it was being read as the
long form, and our code was failing to parse that and just failing when
fetching this from database.
It seems like we aren't the first ones to hit this issue, and serde_json
already supports very large integers.

Here are the docs for this feature:
> Use an arbitrary precision number representation for
serde_json::Number. This
> allows JSON numbers of arbitrary size/precision to be read into a
Number and
> written back to a JSON string without loss of precision.

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server/svix-server/Cargo.toml

@@ -29,7 +29,7 @@ tokio = { version = "1.24.2", features = ["full"] }
 tower = "0.4.11"
 tower-http = { version = "0.3.4", features = ["trace", "cors", "request-id"] }
 serde = { version = "1.0.133", features = ["derive"] }
-serde_json = "1.0.74"
+serde_json = { version = "1.0.74", features = ["arbitrary_precision"] }
 serde_urlencoded = "0.7.1"
 serde_path_to_error = "0.1.7"
 num_enum = "0.5.6"