Cache signatures

[fubuloubu]

Overview

Not sure if this a good idea, but sometimes when you are having to play with a transaction to get it working, you might need to submit it multiple times with different gas values (such as if the gas price keeps rising or something), it would be great in this scenario not to have to call sign_transaction multiple times with the safe, since the request will be the same.

One potential solution is to "cache" the signatures for a given SafeTx hash, since those signatures are likely to continue being valid. You might also want to submit it to the Safe API if it fails the first time, so you can come back and execute it later.

Specification

No UX changes, it just caches by SafeTxID under the hood

Dependencies

n/a

[fubuloubu]

As of #7, the following section was added to the code where a cache could be added:
https://github.com/ApeWorX/ape-safe/pull/7/files#diff-8248524771bdbba6496a80d00a31f24f9d9ec674ae92129bbd2b229d3bc8dbabR278

further research is needed to determine if the cache would be acceptable behavior to users