feat(policy): create subject mappings with attribute values (#3741)

## Summary
- add optional subject mappings to CreateAttributeValue requests
- create inline subject mappings in the same attribute-value transaction
path


<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **New Features**
* Attribute values can now include up to 250 subject mappings,
referencing either existing condition sets or creating new ones inline.
* Creating and retrieving an attribute value now includes the associated
subject mappings with actions, namespaces, and conditions.
* Added request validation for mapping sources, actions, namespaces, and
item limits.
* Enhanced audit logging for subject-mapping and related obligation
trigger events.
* Updated policy documentation (gRPC/HTML and OpenAPI) to reflect the
new request/response shapes.
* **Bug Fixes**
* Prevented attribute value creation when attribute value and subject
mappings use mismatched namespaces.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Signed-off-by: jakedoublev <jake.vanvorhis@virtru.com>

Author: jakedoublev