Add search terms to Command and Signature (#4980)

* Add search terms to command

* Rename Signature desc to usage

To be named uniformly with extra_usage

* Throw in foldl search term for reduce

* Add missing usage to post

* Add search terms to signature

* Try to add capnp Signature serialization

crates/old/nu_plugin_from_sqlite/src/nu/mod.rs

@@ -33,7 +33,7 @@ impl Plugin for FromSqlite {
                 "Only convert specified tables",
                 Some('t'),
             )
-            .desc("Convert from sqlite binary into table")
+            .usage("Convert from sqlite binary into table")
             .filter())
     }