Allow manually provided function.json file

[paulbatum]

See discussion in Azure/azure-functions-host#1507. Basically in the cases where you can't use the attributes (or don't want to) you need to be able to specify function.json directly for a given function.

[ahmelsayed]

I'm adding a file called functionSdk.out that file is a flat list that looks like

function1\function.json
function2\function.json
function3\function.json
....

the logic for crating and using the file is as follows.

                                         +-------------------+
                                         |                   |
                                         |  functionSdk.out  |
                                         |                   |
                                         +-------------------+
                                                   |
                               +----------------------------------------+
                               |                                        |
                               |                                        |
                               v                                        v
                             Exists                               Doesn't Exist
                               +                                         +
                               |                                         |
                               |                                         |
                               v                                         |
                           Load file                                     |
                               +                                         |
                               |                                         |
                               |                                         |
                               v                                         |
                          Delete File                                    |
                               +                                         |
                               |                                         |
                               |                                         |
                               v                                         |
         Delete function.json mentioned in the file                      |
                               +                                         |
                               |                                         |
                               | <---------------------------------------+
                               v
    Create a new empty functionSdk.out for current build
                               +
                               |
                               |
                               v
    Only create a function.json file if it doesn't exist
                               +
                               |
                               |
                               v
     Add function.json to the new functionSdk.out file

[ahmelsayed]

@davidebbo @paulbatum @lindydonna @vijayrkn @mlorbetske
Let me know if you have any concerns with this approach.

[davidebbo]

To put the flowchart's purpose in plain English: "start by deleting any function.json that the tool created in the previous build".