[FEATURE]: buildtest buildspec validate by name

[shahzebsiddiqui]

Please describe your feature

Add option buildtest buildspec validate -n <name> used for validating buildspec by name.

This will be equivalent to how one discovers buildspec via buildtest build --name when building tests.

Suggest potential solution

No response

Additional Information

TODO

• Add option to command line
• Add to bash completion script
• Add implementation in https://github.com/buildtesters/buildtest/blob/devel/buildtest/cli/buildspec.py
• Add regression test in https://github.com/buildtesters/buildtest/blob/devel/tests/cli/test_buildspec.py
• Add documentation in https://buildtest.readthedocs.io/en/devel/gettingstarted/buildspecs_interface.html#validate-buildspecs-buildtest-buildspec-validate

Post question in Slack

• I agree that I posted my question in slack before creating this issue

Is there an existing issue

• I confirm there is no existing issue for this issue