Credential template builder package

This package consolidates all of the CA and SVID template building logic
into one spot where the Credential Composers can be applied.

The package does perform some input validation of parameters but does
unfortunately does not perform validation against alterations returned
by credential composers. This is because it is not possible to know how
the extra extensions will impact the certificates until after they have
passed through CreateCertificate, and we don't want to sign a credential
after each CredentialComposer invocation.

As such, the validation will be left to the server CA layer after
signing has taken place.

Signed-off-by: Andrew Harding <[email protected]>