Implements a variable deposit base calculation for EPM signed submissions

[gpestana]

This PR implements a generic BaseDeposit calculation for signed submissions, based on the size of the submission queue.

It adds a new associated type to EPM's config, type SignedDepositBase, that implements Convert<usize, BalanceOf<T>>, which is used to calculate the base deposit for signed submissions based on the size of the signed submissions queue.

struct GeometricDepositBase<Balance, Fixed, Inc> implements the convert trait so that the deposit value increases as a geometric progression. The deposit base is calculated by deposit_base = fixed_deposit_base * (1 + increase_factor)^n, where n is the term of the progression (i.e. the number of signed submissions in the queue). Fixed and Inc generic params are getters for Balance and IncreaseFactor to compute the geometric progression. If IncreaseFactor = 0, then the signed deposit is constant and equal to Fixed regardless of the size of the queue.

polkadot companion: paritytech/polkadot#7140

Closes https://github.com/paritytech/srlabs_findings/issues/189

[gpestana]

bot rebase

[paritytech-processbot]

Rebased

[Ank4n]

Couple of nits, otherwise LGTM!

[kianenigma]

Moreover I think this is not the case in the code as it stands now yet:

It adds a new associated type to EPM's config type BaseDeposit that implements Convert<usize, Balance>, which is used to calculate the base deposit for signed submissions based on the size of the signed submissions queue.

[kianenigma]

I think this is mainly done, but needs to be brought up to date before a final review.

[gpestana]

bot fmt

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[kianenigma]

can be merged once conflict is resolved.