Retrieves Firebase Project ID from the provided service credential file

[xavarius]

This fixes #142
The pull request adds automatic Firebase Project ID handling by retrieving it from the provided configuration/credential file.
Since the service credential file (exported from the Firebase) contains this project ID, the PR removes duplication in the client initialization.

I would suggest releasing new version with this change, possibly major one, since it's a breaking change.

PS
I have not found any confirmed Firebase service credentials json schema, which kinda makes sense to not disclosure it, but we're all working here with assumptions around the API and file schema.

[xavarius]

@sabman @kashif Could I ask you to review the PR and discuss if needed? :)

[xavarius]

@sabman @kashif Would you be open to including this improvement? Let me know, please. If not, I would consider useing my fork in the project to simplify this bit. :)

[xavarius]

Dear @sabman, I'm fully aware you may need some time to review this. Could you please let me know if you would have opportunity to do it like this month? :)

Thanks a lot!