fix(rpc): set is_cloud_instance remote session var

[greyscaled]

cloud_upload was working via pg proxy but not rpc (see #2714).

I saw that we do this in the pg srv handler:

glaredb/crates/pgsrv/src/handler.rs

Line 261 in d49fbd1

.with_is_cloud_instance(is_cloud_instance, VarType::System);

and tried to find an equivalent spot in RPC code paths, though I'm not 100% on how to grok hybrid exec through the codebase.

---

I deployed this change to QA, and it works on Cloud QA with the CLI, and doesn't seem to unsafely set the cloud instance var:

$ cargo run --bin glaredb -- --cloud-addr=https://qa.glaredb.com
    Finished dev [unoptimized + debuginfo] target(s) in 0.32s
     Running `target/debug/glaredb '--cloud-addr=https://qa.glaredb.com'`
GlareDB (v0.8.5-24-gee80661)
Type \help for help.
> \open glaredb://<connection_string>
Connected to Cloud deployment (TLS enabled): proud_smoke
> show IS_CLOUD_INSTANCE;
┌───────────────────┐
│ is_cloud_instance │
│ ──                │
│ Utf8              │
╞═══════════════════╡
│ false             │
└───────────────────┘
> select * from cloud_upload('idk.csv');
┌─────────────────┐
│ generate_series │
│              ── │
│           Int64 │
╞═════════════════╡
│               1 │
│               2 │
│               3 │
│               4 │
│               5 │
│               6 │
│               7 │
│               8 │
│               9 │
│              10 │
└─────────────────┘
>

[tychoish]

This seems fine, particularly if it works. Is there a way to write a test, or a comment (or set of comments that might prevent this kind of confusion in the future?

[greyscaled]

Is there a way to write a test, or a comment (or set of comments that might prevent this kind of confusion in the future?

I'll take a stab at it, as I agree that it feels uncomfortable with the lack of testing for specific hybrid cases (#2675)