Implement AWS endpoint override option

[jmcruz1983]

Hi there,
currently web3signer implements the possibility to load keys
from AWS secret manager

But it misses the --endpoint-url AWS option for testing purposes,
where remote AWS secret manager is needed
and local testing options like local stack are not possible

I suggest following web3signer cli option:

➜  docker run -it consensys/web3signer:develop eth2 --help
2023-02-28 11:10:46.721+00:00 | main | INFO  | Web3SignerApp | Web3Signer has started with args eth2,--help
2023-02-28 11:10:46.727+00:00 | main | INFO  | Web3SignerApp | Version = web3signer/v21.10.5+124-g17d31bd/linux-x86_64/-eclipseadoptium-openjdk64bitservervm-java-17
Usage: web3signer eth2 [-hV] [--aws-secrets-enabled] [--azure-vault-enabled]
. . . . . 
                       [--aws-endpoint-url=<ENDPOINT_URL>]
. . . . . 
Handle Ethereum-2 BLS signing operations and public key reporting
. . . . . 
      --aws-endpoint-url=<ENDPOINT_URL>
                            AWS endpoint url override.
. . . . . 

For further context please check following forks:

• https://github.com/jmcruz1983/signers/tree/feat-cst-3164
• https://github.com/jmcruz1983/web3signer/tree/feat-cst-3164

Thanks