## Issue Addressed
#3302
## Proposed Changes
Move the `reqwest::Client` from being initialized per-validator, to being initialized per distinct Web3Signer.
This is done by placing the `Client` into a `HashMap` keyed by the definition of the Web3Signer as specified by the `ValidatorDefintion`. This will allow multiple Web3Signers to be used with a single VC and also maintains backwards compatibility.
## Additional Info
This was done to reduce the memory used by the VC when connecting to a Web3Signer.
I set up a local testnet using [a custom script](https://github.com/macladson/lighthouse/tree/web3signer-local-test/scripts/local_testnet_web3signer) and ran a VC with 200 validator keys:
VC with Web3Signer:
- `unstable`: ~200MB
- With fix: ~50MB
VC with Local Signer:
- `unstable`: ~35MB
- With fix: ~35MB
> I'm seeing some fragmentation with the VC using the Web3Signer, but not when using a local signer (this is most likely due to making lots of http requests and dealing with lots of JSON objects). I tested the above using `MALLOC_ARENA_MAX=1` to try to reduce the fragmentation. Without it, the values are around +50MB for both `unstable` and the fix.
Mac L
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