[Feature Request]: Signer support for fee grant module for blob submission

[Bidon15]

Background

Our users want to optimise their cluster operations, thus creating a scenario when 2-4 execution environments (like 4 OPStack chains) are using 1 celestia light node. However, each of the exec env is submitting blobs TXs from their own celestia account. Which brings us to a bad UX, where we have 4 accounts that needs to be funded and tracked.
With fee grant module and signer implementation done on node side, we can eliminate this situation. Meaning that each of the new celestia account for the exec env can be created and granted permissions from a single account to pass fee payments

Implementation ideas

ATM, we need something like a --fee-account $account_address for node's part. This is needed for blob submission

So we can have the following scenario:

1. Light node is initialised/started with an A account with 0 balances
2. The main account (living somewhere else) is granting A account perms to pass the fee payments to it
3. Light node can submit blobs and pass the fees to main account
4. Success for partners running multiple light nodes in their envs

[Bidon15]

https://gist.github.com/rootulp/c47606e177aad1f8959ea31fa077ca5e

[Bidon15]

celestiaorg/celestia-app#3221