solc-json-interface: do not unconditionally skip serialization of custom keys (#337)

The data structure can be used to build the JSON input for `resolc` too.
In that case serializing of provided custom options should not be
dismissed.

Makes the memory settings struct more modular as a drive-by.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

crates/solc-json-interface/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "revive-solc-json-interface"
-version.workspace = true
+version = "0.2.0"
 authors.workspace = true
 license.workspace = true
 edition.workspace = true
@@ -20,4 +20,4 @@ anyhow = { workspace = true }
 rayon = { workspace = true, optional = true }
 semver = { workspace = true }
 serde = { workspace = true }
-serde_json = { workspace = true }
+serde_json = { workspace = true }