instance: Add experimental checkpoint endpointv0.3.3

This commit adds an experimental endpoint that serves the log's to-sign
tree head formatted as a signed checkpoint.  See documentation at:

https://github.com/transparency-dev/formats/tree/main/log#readme

If we decide to adopt this endpoint we should consider in more detail
what the origin string should be.  Right now, it is assumed that the
log's configured prefix is unique across the set of all sigsum logs.

Example output:
```
$ echo "Public verification key: $vk"
Public verification key: 8cf3ac85aadd42891c5ae9aef27244cb2a546a2312f80020aad3f2ae1af73314
$
$ echo "Formatted as a note key: $vkNote"
Formatted as a note key: sigsum.org:testonly+8de2c54b+AYzzrIWq3UKJHFrprvJyRMsqVGojEvgAIKrT8q4a9zMU
$
$ curl http://localhost:6965/testonly/sigsum/v0/get-checkpoint
sigsum.org:testonly
23
HSt6W8ve4/36xAIf04qDOqaKLaKqSOUqKxLNrkK74+g=

— sigsum.org:testonly jeLFS4WQDEeTavbiYArHzCQUPXbQ1Y/V8/dKJlBxqOAY4eLhSh3uWNJ8YXp1vs/zR4SDNHI+6UybLmHmFlg/VSf5OAs=
```

1 files changed, 0 insertions, 18 deletions

diff --git a/issues/add-checkpoint-support.md b/issues/add-checkpoint-support.md
deleted file mode 100644
index f1d7951..0000000
--- a/issues/add-checkpoint-support.md
+++ /dev/null
@@ -1,18 +0,0 @@
-**Title:** Add checkpoint support </br>
-**Date:** 2021-12-09 </br>
-
-# Summary
-Add experimental checkpoint support.
-
-# Description
-Sigsum collaborated on a common
-	[checkpoint format](https://github.com/google/trillian-examples/tree/master/formats/log)
-a while back.  A checkpoint is basically a cosigned tree head.
-
-The current decision is to add experimental support for checkpoints.  There is
-no formal decision to adopt the above checkpoint yet, hence _experimental_.
-
-To keep it simple:
-1. Don't add any timestamp extension.
-2. Only serve the most recent tree head as a checkpoint.  This allows us to
-experiment with external feeders and distributors that are not part of the log.