Add a way to handle int64s serialized as strings

[cmungall]

Some frameworks will serialize what are internally ints as strings in JSON: https://protobuf.dev/programming-guides/proto3/#json

This means that the canonical JSON version of some databases may use strings for things such as genome positions: https://github.com/monarch-initiative/phenopacket-store/issues/97

It would be convenient to have a way of reading/writing these as strings but coercing them to ints.

1. Add a new type, int_as_string
2. add an annotation to the slot that gives serialization/coercion hints
3. handle using linkml-map (a simple slot derivation)

See also

• https://github.com/json-schema-org/json-schema-spec/issues/1395
• https://blog.json-everything.net/posts/numbers-are-numbers-not-strings/