chore: convert DefaultGasAdjustment to var allowing customization of the value

[GAtom22]

Description

Closes: #XXXX

When using the flag --gas auto and the current DefaultGasAdjustment, txs always return an out of gas error. Consequently, to submit the transaction successfully, the user always needs to specify a gas adjustment factor greater than the default.

This PR introduces the change to make this default value a variable, so each project can update this value to fit its needs.

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[julienrbrt]

This seems quite hacky for me.

I'd rather have this implemented: #17253 and have chains set the default in the client.toml.

Happy to remove my R4C if others think otherwise.

[alexanderbez]

Agree with @julienrbrt here. The default gas adjustment is highly dependent on the chain's state anyway. It doesn't always fail.

[julienrbrt]

Thanks for your contribution.
I'll implement #17253 which will solve your issue in a nicer way.