fix: simple ORM for one struct and foreign references work

Currently, all methods declared in the Georm trait are available.

If a struct has an ID pointing towards another entity, the user can
create a get method to get the entity pointed at from the database
too (local one-to-one relationship).

I still need to implement remote one-to-one relationships (one-to-one
relationships when the ID of the remote object is not available
locally).

I still need to also test and debug one-to-many relationships (ID of
the remote entiies not available locally) and many-to-many
relationships (declared in a dedicated table).

For now, IDs in all cases are simple types recognized by SQLx that are
not arrays. Options are only supported when explicitely specified for
one-to-one relationships.

deny.toml

@@ -1,11 +1,12 @@
 [licenses]
-# If there is a need to add another license, please refer to this
-# page: https://www.gnu.org/licenses/license-list.html
+# If there is a need to add another license, please refer to this page
+# for compatible licenses:
+# https://www.gnu.org/licenses/license-list.html
 allow = ["MIT", "Apache-2.0", "BSD-3-Clause", "Unicode-3.0", "Zlib"]
 confidence-threshold = 0.8
 
 [bans]
-multiple-versions = "warn"
+multiple-versions = "allow"
 wildcards = "allow"
 highlight = "all"
 workspace-default-features = "allow"