Implements a variable deposit base calculation for EPM signed submissions

[gpestana]

Note: This is a lift-and-shift PR from the old substrate and polkadot repos, both PRs have been reviewed and audited (paritytech/substrate#13983, paritytech/polkadot#7140)

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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.

Runtime configs

In Kusama, the progression with 10% increase without changing the current signed fixed deposit is: (term == size of the queue)

Term 1: 1,333,333,332,000
Term 2: 1,333,333,332,000 * 1.10 = 1,466,666,665,200
Term 3: 1,333,333,332,000 * 1.10^2 = 1,613,333,331,200
Term 4: 1,333,333,332,000 * 1.10^3 = 1,774,666,664,320
Term 5: 1,333,333,332,000 * 1.10^4 = 1,952,133,330,752
Term 6: 1,333,333,332,000 * 1.10^5 = 2,147,346,663,827.20
Term 7: 1,333,333,332,000 * 1.10^6 = 2,362,081,330,210.92
Term 8: 1,333,333,332,000 * 1.10^7 = 2,598,289,463,231.01
Term 9: 1,333,333,332,000 * 1.10^8 = 2,858,118,409,554.11
Term 10: 1,333,333,332,000 * 1.10^9 = 3,143,930,250,509.52

Westend:

Term 1: 2,000,000,000,000
Term 2: 2,000,000,000,000 * 1.10 = 2,200,000,000,000
Term 3: 2,000,000,000,000 * 1.10^2 = 2,420,000,000,000
Term 4: 2,000,000,000,000 * 1.10^3 = 2,662,000,000,000
Term 5: 2,000,000,000,000 * 1.10^4 = 2,928,200,000,000
Term 6: 2,000,000,000,000 * 1.10^5 = 3,221,020,000,000
Term 7: 2,000,000,000,000 * 1.10^6 = 3,543,122,000,000
Term 8: 2,000,000,000,000 * 1.10^7 = 3,897,434,200,000
Term 9: 2,000,000,000,000 * 1.10^8 = 4,287,177,620,000
Term 10: 2,000,000,000,000 * 1.10^9 = 4,715,895,382,000

and in Polkadot, the deposit increase is disabled in the current state of the PR, as the increase factor is 0% -- so nothing changes from the current behaviour.

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

[gpestana]

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