Make RdfSource generic over the dataset type

Cargo.lock

@@ -244,9 +244,9 @@ dependencies = [
 
 [[package]]
 name = "either"
-version = "1.15.0"
+version = "1.16.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "48c757948c5ede0e46177b7add2e67155f70e33c07fea8284df6576da70b3719"
+checksum = "91622ff5e7162018101f2fea40d6ebf4a78bbe5a49736a2020649edf9693679e"
 
 [[package]]
 name = "errno"
@@ -687,6 +687,7 @@ dependencies = [
  "oxigraph",
  "parse_link_header",
  "reqwest-middleware",
+ "slotmap",
  "thiserror",
  "tracing",
 ]
@@ -1308,6 +1309,15 @@ version = "0.4.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0c790de23124f9ab44544d7ac05d60440adc586479ce501c1d6d7da3cd8c9cf5"
 
+[[package]]
+name = "slotmap"
+version = "1.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bdd58c3c93c3d278ca835519292445cb4b0d4dc59ccfdf7ceadaab3f8aeb4038"
+dependencies = [
+ "version_check",
+]
+
 [[package]]
 name = "smallvec"
 version = "1.15.1"