feat(x/provider): lock fees_per_block denom at module wiring

[giunatale]

Closes #49

Follow-up to #47. Params.FeesPerBlock (a sdk.Coin) let MsgUpdateParams silently change the fee denom, which would break downtime slashing logic and force an upgrade of any per-consumer override.

This PR removes the denom from params entirely: the amount stays a param (Params.FeesPerBlockAmount, math.Int), while the denom is fixed at keeper construction (Keeper.feeDenom) and cannot change without a binary upgrade — satisfying the constitution's "ICS payments in PHOTON" requirement.

The reference app wires providertypes.DefaultFeesPerBlockDenom while AtomOne is supposed to wire photontypes.Denom. Both resolve to uphoton but the former is less formal and just for testing purposes. NewKeeper panics on an invalid denom.

BREAKING: Params proto field 5 changes from sdk.Coin fees_per_block to string fees_per_block_amount.

[giunatale]

#46 would require a minor change/adaptation due to this PR (i.e. params.FeesPerBlock.Denom to k.GetFeeDenom(ctx) when checking for denom correctness upon fee pool funding). If this PR gets merged first I'll fix it in #46 after merge, otherwise I'll fix it either in this PR or in a followup

[tbruyelle]

LGTM

Tiny optimization: DistributeFeesToValidators() should call k.GetFeeDenom() instead of k.GetFeesPerBlock().Denom to prevent a useless params store read.

Additional note: impact the wiring in atomone-hub/atomone#283 (cc @julienrbrt)

[giunatale]

Tiny optimization: DistributeFeesToValidators() should call k.GetFeeDenom() instead of k.GetFeesPerBlock().Denom to prevent a useless params store read.

fair point!

[tbruyelle]

Just realized as well that make provider-start for localnet should be updated just like the e2e/provider-init.sh with the add-genesis-account command.