feat: add Stader validator registry support

[PedroCM96]

Add support for loading validator keys from Stader registries.

A new stader registry is introduced with explicit pool selection for Stader's permissioned or permissionless node registries.

This implementation follows the same high-level structure used for the Lido implementation: resolve the registry contract, fetch the total number of keys and load those keys in batches through the shared registry key collection logic.

[ManuelBilbao]

Please update the loader reference in config.example.toml

commit-boost-client/config.example.toml

Lines 139 to 160 in 635384f

	# Loader for validator pubkeys. Three types of loaders are supported:
	# - File: path to a file containing a list of validator pubkeys in JSON format
	# - URL: URL to an HTTP endpoint returning a list of validator pubkeys in JSON format
	# - Registry: details of a registry to load keys from. Supported registries:
	# - Lido: NodeOperatorsRegistry
	# - SSV: SSV API
	# You can toggle the 'enable_refreshing' flag to let this registry periodically query Lido or SSV and refresh the list of validator pubkeys belonging to the corresponding operator.
	# Each of these registry entries must be unique:
	# - There can only be one Lido entry with a given Lido node operator ID.
	# - There can only be one SSV entry with a given SSV node operator ID.
	# - A Lido entry can have the same node operator ID as an SSV entry if they happen to coincide; they're treated as separate entities.
	#
	# Example JSON list:
	# [
	# "0x80c7f782b2467c5898c5516a8b6595d75623960b4afc4f71ee07d40985d20e117ba35e7cd352a3e75fb85a8668a3b745",
	# "0xa119589bb33ef52acbb8116832bec2b58fca590fe5c85eac5d3230b44d5bc09fe73ccd21f88eab31d6de16194d17782e"
	#]
	# OPTIONAL
	loader = "./tests/data/mux_keys.example.json"
	# loader = { url = "http://localhost:8000/keys" }
	# loader = { registry = "lido", node_operator_id = 8, lido_module_id = 1, enable_refreshing = false }
	# loader = { registry = "ssv", node_operator_id = 8, enable_refreshing = false }