soroban-rpc: make preflight fudge factors configurable by the client

[mollykarcher]

What problem does your feature solve?

Preflight estimations have been off, requiring us to repeatedly issue releases to up the fudge factor used in these calculations. When users are unable to create contracts due to these issues, there aren't any good workarounds for them.

What would you like to see?

The preflight request will accept new parameters that translate into the fudge factor used by preflight. TBD whether we should support/add both the percentage and flat addition factors, or if we should just support one or the other. End users could then tweak this up or down depending on their specific needs. These parameters should be optional and the default should be the current hardcoded values. Optionally, we could make these defaults configurable RPC/server side as well.

What alternatives are there?

Alternatively discussed making the fudge factor(s) configurable RPC-side. This would still allow us and other RPC providers to move the fudge factor without having to release/deploy. However, given a single RPC instance will service many different profiles of dapps/users, and it's impossible to provide a one-size-fits-all default that will work for all contracts, it makes more sense for it to be client provided.