Encapsulate IMA policies in DSSE envelopes

[mbestavros]

Enhancement Description

• One-line enhancement description (can be used as a release note): Encapsulate IMA policies in DSSE envelopes
• Keylime Enhancement Proposal: (link to keylime/enhancements file, if none yet, link to PR)
• Primary contact (assignee): @mbestavros
• Enhancement target (which target equals to which milestone):
  • Alpha release target (x.y)
  • Beta release target (x.y)
  • Stable release target (x.y)

Originally suggested by @lukehinds in #77. With the emergence of upstream standards for signing envelopes, it may make more sense to use those standards instead of a Keylime-specific envelope format.