radicle: fix schemars macro on FetchPackSizeLimit — heartwood

While running cargo-semver-checks, there was an issue that was surfaced regarding serde(into), serde(try_from) and schemars(transparent) being used, and they are incompatible.

This appeared to be a semver issue, on the side of schemars, since the issue would only appear when adding radicle as a fresh dependency. The intuition for this is that once schemars was updated locally, the same error appeared when attempting to build the radicle crate.

To avoid this, schemars provides a ! operator for the macro¹:

You can also “unset” serde attributes by including them with a ! prefix in a schemars attribute, which will make schemars ignore the corresponding serde attribute item

¹

https://docs.rs/schemars_derive/latest/schemars_derive/derive.JsonSchema.html#attributes

 checksum = "778e2ac28f6c47af28e4907f13ffd1e1ddbd400980a9abd7c8df189bf578a5ad"
 dependencies = [
  "libc",
  "windows-sys 0.52.0",
  "windows-sys 0.59.0",
 ]
 [[package]]

  "portable-atomic",
  "portable-atomic-util",
  "serde",
  "windows-sys 0.52.0",
  "windows-sys 0.59.0",
 ]
 [[package]]

  "errno",
  "libc",
  "linux-raw-sys 0.9.4",
  "windows-sys 0.52.0",
  "windows-sys 0.59.0",
 ]
 [[package]]

 [[package]]
 name = "schemars"
 version = "1.0.0-alpha.17"
 version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "88ef2a6523400a2228db974a8ddc9e1d3deaa04f51bddd7832ef8d7e531bafcc"
 checksum = "82d20c4491bc164fa2f6c5d44565947a52ad80b9505d8e36f8d54c27c739fcd0"
 dependencies = [
  "dyn-clone",
  "ref-cast",

 [[package]]
 name = "schemars_derive"
 version = "1.0.0-alpha.17"
 version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c6d4e1945a3c9e58edaa708449b026519f7f4197185e1b5dbc689615c1ad724d"
 checksum = "33d020396d1d138dc19f1165df7545479dcd58d93810dc5d646a16e55abefa80"
 dependencies = [
  "proc-macro2",
  "quote",

 radicle-ssh = { path = "crates/radicle-ssh", default-features = false }
 radicle-systemd = { path = "crates/radicle-systemd" }
 radicle-term = { path = "crates/radicle-term" }
 schemars = { version = "1.0.0-alpha.17" }
 schemars = { version = "1.0.4" }
 serde = "1.0"
 serde_json = "1.0"
 shlex = "1.1.0"

 #[cfg_attr(
     feature = "schemars",
     derive(schemars::JsonSchema),
     schemars(transparent)
     schemars(transparent),
     // serde's transparent and try_from/into will conflict, so we tell schemars
     // to ignore them for its generation.
     schemars(!try_from),
     schemars(!into),
 )]
 pub struct FetchPackSizeLimit {
     #[cfg_attr(