Update handler name when deploy a single function

[bzemms]

What did you implement:

In this PR I fixed an issue (or a not implemented feature), according the function handler name of an AWS Lambda, while deploying a single function. (using deploy -f / --function)

Former to that fix, when deploying a single function even if you change the code and the handler file / handler method , changing the handler name in the serverless.yml was not affected.

(BTW other configuration such as memorySize, description, timeout, etc.. are affected as expected)

Closes #XXXXX

How did you implement it:

As done for others configuration ( such as memorySize, description, timeout, etc..)
while updateFunctionConfiguration() {...} in /lib/plugins/aws/deployFunction/index.js - I did implement the same logic to take the handler name from the functionObj and added it to params of the callUpdateFunctionConfiguration method

How can we verify it:

1. Deploy a simple sls stack with one function to aws.
2. Change the handler name in the serverless.yml (e.g to point on an other method)
3. Deploy this single function using -f / --function cmd line option.
4. Check in the AWS Lambda console that the new handler name appear in the configuration

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[iharush]

+1 Will be very helpful

[horike37]

@bzemms
Great work 👍 It works as expected on my end.
I think this is a promising solution 🥇 LGTM.