[Vault-5248] MFA support for api login helpers

[VinnyHC]

This PR adds MFA support for auth methods (login helpers).

Single Phase MFA with Login Helper
Pass in credentials using same format as (*client).SetMFACreds() as variadic parameter. This is a simple convenience approach around our existing code.

// existing code
(*client).SetMFACreds(fmt.Sprintf("%s:%s", methodID, passcode))
(*client).Auth().Login(context.Context, AuthMethod)

// PR simplification
(*client).Auth().MFALogin(context.Context, AuthMethod, fmt.Sprintf("%s:%s", methodID, passcode))

Two Phase MFA with Login Helper
Flow when no credentials are provided. Performs checks on secret to confirm it contains MFARequirements for follow up call.

mfaSecret, err := (*client).Auth().MFALogin(context.Background(), &AuthMethod)
mfaPayload := map[string]interface{}{
  "method-id": []string{"a-passcode"},
}
authSecret, err := (*client).Auth().MFAValidate(context.Background(), mfaSecret, mfaPayload) 

This PR also adds the new method (*client).Sys().MFAValidateWithContext() a convenience method for calling sys/mfa/validate.

[averche]

Looks really good, left a couple small comments

[digivava]

Isn't the only reason we provide a difference between MethodName and MethodNameWithContext because we're trying to maintain backwards compatibility for users who are already using the one that doesn't take a context?

In this case, this is a brand new method, so I thought we would just want to have MFAValidate take a context as its first arg.

[VinnyHC]

I could be convinced either way!
The motivation here was that this lives in the existing (*client).Sys() space where most methods have both MethodName and MethodNameWithContext whereas (*client).Auth() is the opposite (outside of (*client).Auth().Token()), so I wanted (*client).Auth().MFALogin() to be as close to the existing (*client).Auth().Login() as possible.

[raskchanky]

Looks great! I don't have anything to add beyond what's already been commented on. Thanks for updating the existing tests to use the new methods here!

[averche]

LGTM, awesome job!