Get usage data for metered service dimensions

What we're after

The services in #3285 are all gathered from querying CAPI. They are billed based on their presence or absence — i.e. the user is billed for the period of time they are running. "Metered" services are billed based on a different usage dimension than present/absent. For example:

• RDS disk size is billed based on GB allocated over time — GB-month. This information is available in the service parameters or in AWS.
• S3 buckets are billed based on GB storage consumed — also GB-month. This information is available in AWS.
• SES is billed by emails sent over a given period of time.

The goal of this issue is to read usage information from these services.

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Further context for those unfamiliar with what we're doing

• Pricing model or cost estimator enumerates all service rates.

Security considerations

Getting usage information for some services will require interconnection with, and authentication to, AWS.

Notes for implementers

• This likely requires enqueuing additional work after taking the preliminary readings. I.e. we read an S3 bucket from CAPI and enqueue a job to read further data from AWS.
  • Alternatively, we could read data directly from AWS based on tags, but we probably want to use CAPI as the source of truth — basically, "left join" CAPI and AWS data.
• Like the CAPI ticket, bulk requests will be our friend here. N+1 will kill performance.

Related issues/sub-projects

• #3285