Parameterize stake to weight conversion

[cam-schultz]

Context and scope
The weight to value ratio is hardcoded at 1 weight per 1e12 units of value (e.g. ERC20 or native token base denominations). This hardcoded ratio assumes that validators added from sources other than PoSValidatorManager (e.g. a PoAValidatorManager that is migrated to PoSValidatorManager, or the initial validator set specified in the P-Chain ConvertSubnetTx) use weights that translate to reasonable asset amounts.

Instead, PoSValidatorManager should parameterize this relationship such that future validator set changes can use reasonable stake amounts regardless of the initial validator weights.