Add initial proposal to support hcl + yaml as configuration files

[MarcosDY]

Changes

• Adds experimental YAML as an optional configuration format for SPIRE server and agent, detected by file extension  (.yaml/.yml). HCL behavior is completely unchanged.
• Introduces pkg/common/plugindecode.DecodeConfig as a format-aware decode helper used by all built-in plugins.
• Updates DataSource to carry ConfigFormat alongside config text
• Update plugins.

Notes

• Experimental: A warning is logged when a YAML config file is loaded.
• spire-plugin-sdk dependency: This PR requires a companion PR to spire-plugin-sdk to be merged.
• No changes to existing HCL parsing — all current config files continue to work without modification.