enhance: support map type for build args

[ecrupper]

These changes will allow the following pipeline:

steps:
  - name: kaniko
    image: target/vela-kaniko:latest
    parameters:
      build_args:
        list: one,two,three
        foo: bar
      dry_run: true
      registry: docker.company.com
      repo: docker.company.com/myapp

Today, if using the slice input, the above would interpret the build args as --build-arg list=one --build-arg two --build-arg three --build-arg foo=bar.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 34.88%. Comparing base (abe96d6) to head (51374e0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
- Coverage   35.55%   34.88%   -0.67%     
==========================================
  Files           7        7              
  Lines         526      536      +10     
==========================================
  Hits          187      187              
- Misses        339      349      +10     

Files	Coverage Δ
cmd/vela-kaniko/main.go	0.00% <0.00%> (ø)

[wass3rw3rk]

thanks for maintaining backwards compatibility