cache credential bundle instead of reading from file path on each request #459

In internal/credbundle/credbundle.go , we should cache the credential bundle instead of reading from file path on every request.

References:

• PodIdentity signer issues certs that are valid for 24 hours:

  substrate/cmd/podcertcontroller/internal/podidentitysigner/podidentitysigner.go

  Line 108 in 381b3ad

  lifetime := 24 * time.Hour