Parse SpiceDB Schema DSL to EACL Datomic Entities

[theronic]

WIP.

Do we need to support multiple nested arrow permissions? e.g. permission admin = account->thing->admin. We don't currently support this internally since it can be modelled with permission on multiple resource definitions.

[theronic]

Addressed in #55

Implementation details:

• Added instaparse dependency for parsing SpiceDB schema DSL
• eacl.datomic.spice-parser implements a full SpiceDB grammar that parses the complete official syntax
• parse-schema parses DSL strings into parse trees
• ->eacl-schema transforms parse trees into EACL internal representation ({:relations [...] :permissions [...]})
• EACL-specific restrictions (no intersection/exclusion operators, no multi-level arrows, no wildcards, etc.) are enforced during validation with informative error messages
• Regarding nested arrows: multi-level arrows like a->b->c are parsed but rejected during validation, as noted in the issue - these can be modeled with permissions on multiple resource definitions