Make base extrinsic weight stable for capacity txs

[enddynayn]

Description
Capacity transactions are meant to be stable and not affected by network congestion.

Additionally, unlike token transactions, capacity transactions
do not allow tipping or multiplying the weight
fee by a targeted_fee_adjustment.

The fee for a capacity transaction:

fee = base_fee + length_fee + weight_fee
----- ^^^^
----- ^^^^

The base_fee is not static and can change when we run overhead benchmarks.

Acceptance Criteria:

• Ensure that the base_extrinsic weight does not change for Capacity transactions.
• Add comment to weight to fee function to warn that it also changes static capacity weights if changed.

Note
Weights for capacity are a
snapshot of weights calculated from benchmarks.
These weights are static and are not updated.
Similarly, we would want to create a constant containing a snapshot of
extrinsic base weight.

@wilwade check