Support builder boost factor in proposer settings file

[nflaig]

Problem description

We are not supporting boostFactor in proposer settings file but instead only on the keymanager API

lodestar/packages/cli/src/util/proposerConfig.ts

Lines 14 to 24 in e51f495

	type ProposerConfigFileSection = {
	graffiti?: string;
	strict_fee_recipient_check?: string;
	fee_recipient?: string;
	builder?: {
	// boolean are parse as string by the default schema readFile employs
	// for js-yaml
	gas_limit?: number;
	selection?: routes.validator.BuilderSelection;
	};
	};

Solution description

Add builder.boostFactor to proposer settings file to allow per validator configuration

Additional context

No response

[nflaig]

Needs to apply same validation as CLI arg --builder.boostFactor added in #6332.