Add spiffe-inspect test command to tbot

[strideynet]

A small command that can be pointed against a Workload API endpoint to ensure it works correctly. I realised this would be useful in the docs so we can assure people they have set up tbot correctly before moving on to configure their application.

changelog: Adds tbot spiffe-inspect for testing the configuration of Teleport Workload Identity.

teleport git:(strideynet/spiffe-tester-tbot) ✗ build/tbot spiffe-inspect --path unix:///Users/noah/code/gravitational/workload-identity-experiment/workload.sock
INFO [TBOT]      Inspecting SPIFFE Workload API Endpoint path:unix:///Users/noah/code/gravitational/workload-identity-experiment/workload.sock tbot/spiffe.go:31
INFO [TBOT]      Received X.509 SVID context from Workload API bundles_count:1 svids_count:2 tbot/spiffe.go:46
SVIDS
- spiffe://leaf.tele.ottr.sh/bar
  - Hint: test-1
  - Expiry: 2024-03-20 10:55:52 +0000 UTC
  - DNS SAN: example.com
  - DNS SAN: foo.example.com
  - IP SAN: 14.1.1.1
- spiffe://leaf.tele.ottr.sh/foo
  - Hint: test-2
  - Expiry: 2024-03-20 10:55:52 +0000 UTC
  - DNS SAN: example.com
  - IP SAN: 14.1.1.1
Trust Bundles
- leaf.tele.ottr.sh

[public-teleport-github-review-bot]

@strideynet See the table below for backport results.

Branch	Result
branch/v15	Create PR