This allows you to create an entry for the entire certificate chain, not
just the leaf certificate. The certificate chain will be verified before
adding the entry.

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>

* use an in memory timestamping key

Signed-off-by: Asra Ali <asraa@google.com>

* address comments

Signed-off-by: Asra Ali <asraa@google.com>

* Adopt new signing/verification APIs from sigstore

This uses the new APIs introduced in sigstore/sigstore/pkg/signature and
removes most of the calls directly to the golang crypto APIs.

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* Add timestamps

Signed-off-by: Asra Ali <asraa@google.com>

* change

Signed-off-by: Asra Ali <asraa@google.com>

* address comments

Signed-off-by: Asra Ali <asraa@google.com>

* address comments

Signed-off-by: Asra Ali <asraa@google.com>

* fix binary writer

Signed-off-by: Asra Ali <asraa@google.com>

* add tsa

Signed-off-by: Asra Ali <asraa@google.com>

* distangle cert chain creation from new signer

Signed-off-by: Asra Ali <asraa@google.com>

* revert some now unncessary changes

Signed-off-by: Asra Ali <asraa@google.com>

* cert chain 404

Signed-off-by: Asra Ali <asraa@google.com>

* fix

Signed-off-by: Asra Ali <asraa@google.com>

* update boilerplate header and apply go fmt

Signed-off-by: Carlos Panato <ctadeu@gmail.com>

* lints: fix golangci-lint issues

Signed-off-by: Carlos Panato <ctadeu@gmail.com>

* updated based on feedback

Signed-off-by: Carlos Panato <ctadeu@gmail.com>

Still need to add some more e2e-style tests that go through the ArtifactFactory flow.