Add a new feature to spire-server to print to standard output the server's bundle in PEM format

[martincapello]

There are some situations where we need the server's CA bundle, for instance when:

• Bootstrapping new agents.
• We want to expose bundle management to the user.

To fulfill this requirement a new command/sub-command must be added to spire-server which usage is detailed as follows:

$ spire-server bundle show

This must print to standard output the server's CA bundle in PEM format.