Directly Run Credentials Process

[ijrsvt]

• Directly call credential process instead of /bin/sh -c "credential process + args
• This more directly follows the botocore variant. https://github.com/boto/botocore/blob/409b4043602eacfd0951ea1b28aa52c5f6dbb6bd/botocore/credentials.py#L1052
• Makes using credential_process usable in distroless containers (which lack /bin/sh)

[skmcgrail]

Sorry we can't accept this pull request due to several issues:

• This would be a backwards incompatible change in behavior users who expect that the shell is being loaded for the sourcing of environment configuration. This would need minimally be gated behind some sort of configuration value to toggle the desired behavior. Likely something that can be toggled programmatically when calling config.LoadDefaultConfig
• The given implementation lacks parsing/lexing of the command necessary to support commands that take arguments. Currently the way this is invoking the process it would pass the arguments as a single argument to the process, which would be incorrect.
• Windows platforms would also need their own level of special handling of command argument parsing that are specific to how commands are pared there.

[skmcgrail]

See this issue that was originally cut towards the Go repository #2879 which covers tracking this as a cross-sdk issue, as this is not just limited to the Go SDK.