Dual TLS certification chain and crypto agility support

[ansasaki]

Enhancement Description

• One-line enhancement description: Add support for dual TLS certification chain and crypto agility
• Keylime Enhancement Proposal: (link to keylime/enhancements file, if none yet, link to PR)
• Primary contact (assignee): @ansasaki
• Enhancement target (which target equals to which milestone):
  • Alpha release target (x.y)
  • Beta release target (x.y)
  • Stable release target (x.y)