Fixed project level billing budget filter

[kovagoadam]

Modified budget_filter for billing budget

budget_filter resource does not take project_id instead it needs the project number.

Related to #3553 #3554, but that is for project factory and this is for the billing budget itself.

Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

[ludoo]

local.ctx will not contain the internally defined project numbers, you have to merge them in first.

Check https://github.com/GoogleCloudPlatform/cloud-foundation-fabric/blob/master/modules/project-factory/projects.tf#L51 for comparison

[ludoo]

I also don't think we should change the semantics of the project numbers context to have a projects/ prefix: the billing module should interpolate that. We can't expect the caller to know if project numbers should be plain numbers (like in all other modules), or prefixed.

I would do something like this in the billing module instead: try("projects/${local.ctx.project_numbers[v]}", v)