Refactor invited groups for billable user methods

What does this MR do and why?

• moves the project_group_link and group_group_link queries into scopes for better testing/ability for reuse

Query analysis using gitlab-org id

query	plan
billed_group_users	31156 54ms
billed_project_users	22112 44ms
billed_shared_group_users	33432 54ms
billed_invited_group_to_project_users	47892 74ms

Old query analysis before removing index change from this MR

Query analysis using gitlab-org id

query	before	with correct users index(doug_index)
billed_group_users	31156 ms	31248 54ms
billed_project_users	22112 44ms	21867 40ms
billed_shared_group_users	33432 54ms	33429 59ms
billed_invited_group_to_project_users	47892 74ms	47884 81ms

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383264