Provide the ability to perform offline, post-facto attestation on agents (nodes) long retired

# Enhancement Description
- One-line enhancement description: provide keylime with the ability to dump all the attestation information in a time series-like datastore, which could then be used to offer proof that a certain agent was at a given state, at a given date.
- Keylime Enhancement Proposal: TBD
- Primary contact: @maugustosilva
- Enhancement target (which target equals to which milestone):
- Alpha release target (6.4.1)
- Beta release target (6.4.X)
- Stable release target (TBD)

_Please to keep this description up to date._

Start with the enhancement description and the currently TBD Keylime Enhancement Proposal; no implementation files, tests, or entry points are identified. Clarify the historical attestation data and proof requirements, then define the scope and acceptance criteria for storing time-series information about retired agents.

Written by the indexing model from the issue text.