feat (InjectScript): Provide way to conditionally inject script

[jafin]

Introduce prop injectScript: boolean

• Provide way to conditionally inject script should help with #30 .
• Provides the ability to delay script injection for cases such as not having the Id at time of render GTMId (ie pulled in via REST) but still having the Provider Component rendered.
• SendToGTM will check for window.Datalayer or warn if not found, as it is not guaranteed now to have the script loaded.

The default value is true, to avoid changing existing API surface.

[tcarter8]

I have a need to inject the GTM script only after the home page video has loaded, for performance reasons. Can anyone provide an example of how this injectScript prop can be used? If it provides a way to delay script injection, I can find no examples. It's obvious injectScript: false would be declared in the initial gtmParams. But I can't visualize how I would tell the GTMProvider that injectScript should toggle to true after getting my video onLoadedData event. Thanks in advance!