feat(proto): Wire custom SchemaHandler for proto field metadata

[jamesarich]

Summary

Adds a build-time Wire SchemaHandler that auto-generates a ConfigFieldMetadataRegistry from (meshtastic.config_field) annotations in .proto files.
Companion to: meshtastic/protobufs#905

Problem

Jonathan's protobufs PR #905 adds custom field options (diy_only) to proto fields via extend google.protobuf.FieldOptions. The question was whether Android (Wire) could access this metadata without hacks.

Solution

Wire 6 handles proto extensions cleanly — it flattens them into typed properties on FieldOptions. However, the metadata lives at the schema level, not on generated message fields directly. To make it easily queryable at runtime without shipping the heavy wire-schema library in the APK, we use Wire's custom SchemaHandler mechanism:

1. FieldMetadataSchemaHandler (in build-logic/convention) walks every message field at codegen time
2. Reads (meshtastic.config_field) options via field.options.get(ProtoMember)
3. Emits a static ConfigFieldMetadataRegistry Kotlin object

Generated output (auto-created at build time):

object ConfigFieldMetadataRegistry {
    fun isDiyOnly(messageType: String, fieldTag: Int): Boolean
    fun diyOnlyFields(messageType: String): Set<Int>
}

Usage example:

// In PositionConfigScreen — gate GPIO settings for DIY devices only
if (!ConfigFieldMetadataRegistry.isDiyOnly("meshtastic.Config.PositionConfig", 8) || isDiy) {
    DropDownPreference(title = stringResource(Res.string.gps_receive_gpio), ...)
}

Key properties

• Zero runtime cost — static map, no reflection, no descriptor parsing
• Auto-discovery — annotating new proto fields requires zero code changes
• No app-size impact — wire-schema is only a build-logic dependency, not shipped in APK
• Only modify handler if ConfigFieldMetadata message gains new sub-fields (e.g. admin_only, min_value)

Changes

• gradle/libs.versions.toml — added wire-schema library entry
• build-logic/convention/build.gradle.kts — added wire-schema dependency
• build-logic/convention/.../FieldMetadataSchemaHandler.kt — the custom handler + factory
• core/proto/build.gradle.kts — added custom {} target + source dir registration