Validate application name

[efekarakus]

Story

As a user that entered the app name to the ecs init prompt or flag,
I want the CLI to give me an error before it proceeds creating my resources,
So that I don't do additional wasted work before getting an error message.

Acceptance Criteria

• Has to be less than 255 characters

This is because an app's name will be used as a tag value which has to be < 255 chars. Note: I'm not sure what this means for unicode characters we might want to test this out.

• Must be alphanumeric (A-Za-z0-9)

We will use a combination of project name and application name to create the Logical ID of CFN resources. CFN has a restriction of alphanumeric ids.

• Must be unique within the project.

Maybe we should hold onto the SDK validations until we have the project management steps done.

[sonofachamp]

Is checking for uniqueness within the project a local check of the Manifest file?

[kohidave]

@sonofachamp - It depends on how our final project management approach, but I imagine we'll do a uniqueness check by querying SSM or a CP to see if the app already exists.

[sonofachamp]

I've opened #19 to add some basic value validations and will open another issue to track the uniqueness within a project scope work.