[FEATURE] Implement tag assignment values

[roryqi]

Describe the feature

Implement assignment-level values for tags so one metadata object can assign multiple values to the same tag.

Motivation

Current tag assignment is boolean. Users need searchable tag values such as data_domain=finance without creating one tag per value.

Describe the solution

• Add allowedValues to tag creation and tag DTOs.
• Change tagsToAdd and tagsToRemove to tag-value pair arrays.
• Store one value per tag_relation_meta row with tag_value and value_order.
• Make repeated assignment idempotent and use exact value lookup through GET /tags/{tag}/objects?value=....

Additional context

Initial implementation does not support modifying allowedValues after tag creation.