Add dedicated `FeePolynomial` struct (#13612)

* Add FeePolynomial struct

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

* Add Weight::without_{ref_time, proof_size}

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

* Docs

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

* Cleanup code

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

* Add docs

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

* doc

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

* Fix docs

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

* docs

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>