AWS CI pipeline

[phbnf]

TODO - before sending it for a review:

• setup the right permission for the trillian-tessera acount
• update the repository and branches referenced in the workflow file

Towards #312.

This PR introduces a CI pipeline for AWS Tessera running on GitHub actions.

On every push to main, a GitHub action:

1. authenticates with the trillian-tessera AWS account using secret keys stored on GitHub
2. builds the conformance and hammer docker images
3. pushes them to a container registry on AWS
4. brings down old ci environments if need be
5. generates Tessera keys
6. brings up a new ci environment with:
   • an Aurora RDS DB
   • an S3 bucket
   • an ECS cluster
   • an ECS Fargate service running multiple instances of the conformance binary
   • it defines (but does not launch) an ECS Fargate hammer task
7. launches the hammer task against the conformance service, and waits for it to return
8. brings down the full task

The whole process takes around 30 minutes to complete, mostly since RDS databases take between 5 to 10 minutes to be brought up and down. Here's an example run. A concurrency blocks prevents two workflows to run simultaneously.