CLI: specify ATTESTER to build kbs-client

[genjuro214]

We can't build kbs-client on s390x host with default feature including all attesters.

I tried to add a new variable ATTESTER in Makefile, so that we can specify separate attester when building kbs-client

• by default, make cli will build kbs-client with all attesters
• keep feature sample_only: CLI_FEATURES=sample_only make cli
• specify ATTESTER: ATTESTER=se-attester make cli

[huoqifeng]

se-only feature tag looks weird to me, maybe, considering to add ATTESTER like guest-components? like https://github.com/confidential-containers/guest-components/blob/main/attestation-agent/Makefile#L28

[huoqifeng]

@Xynnn007 @fitzthum @arronwy may you help have a look?