Feature/support slice and map structs

[Azhovan]

What

Add partial nested collection binding support for struct elements/values in the existing load/bind flow:

• Bind []T where T is a struct from YAML-decoded collection values (for example, []any containing nested objects).
• Bind map[string]T where T is a struct when the source provides a direct nested map value (for example, in-memory/custom sources returning map[string]any values).
• Add loader regression tests covering successful binding and invalid nested type conversion (invalid_type) for nested collections.

This PR does not yet support binding map[string]T from flattened dotted keys (for example, clickhouse_map.primary.host) produced by the built-in file source, and does not include strict-mode dynamic nested-key validation for that shape.

Docs: N/A — this change does not alter public APIs, config tags/defaults, examples, or setup/onboarding flow.

Why

Rigging already supports nested struct binding, but nested collections of structs ([]T, map[string]T) were not handled in the binder for direct collection values. This PR delivers a minimal, low-risk improvement while keeping scope small.

Type

• Fix
• Feature
• Docs
• Performance
• Breaking change

Partially addresses #33

[Copilot]

Pull request overview

This PR adds support for binding nested collections of structs ([]T and map[string]T where T is a struct) from YAML-decoded configuration values. It enables common configuration patterns like multiple database connections or service endpoints to be defined as slices or maps of structured configuration objects.

Changes:

• Enhanced convertValue in binding.go to recursively handle slices and maps with struct element/value types
• Added helper functions convertCollectionElement and convertStructValue to properly bind struct instances within collections
• Added comprehensive test coverage in loader_test.go for successful binding and error cases

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
loader_test.go	Adds TestLoad_NestedCollections with subtests for binding slices/maps of structs and validating type conversion errors
binding.go	Extends convertValue to handle slices/maps of any type including structs, adds convertCollectionElement and convertStructValue helpers for recursive binding

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