CertifyKey stack usage is incredibly high

[jhand2]

I was looking at the CertifyKey disassembly and it uses several KiB of stack (as of this writing CertifyKeyCmd::execute uses almost 7KiB of stack).

This is somewhat expected since it needs to be able to generate a several KiB cert, but we should see if we can optimize this a bit. There are lots of temporary byte arrays that we use as scratch space and could maybe eliminate. This will get worse when we go to increase the max cert size (#357), so we should address this now.

[jhand2]

For example, this is a pretty big allocation:

caliptra-dpe/dpe/src/commands/certify_key.rs

Lines 124 to 125 in 80c5ed6

	const INITIALIZER: TciNodeData = TciNodeData::new();
	let mut nodes = [INITIALIZER; MAX_HANDLES];

Instead, we could consider making an iterator that just gets references to the underlying contexts.