fix: Misc meltano.yml schema fixes

[WillDaSilva]

• Mappers now support the variant field.
• Extractors may now have an env field within the environments field.

Tested with the following JSON at https://jsonschemalint.com/#!/version/draft-07/markup/json:

{
  "plugins": {
    "mappers": [
      {
        "name": "mapper",
        "variant": "mapper-variant"
      }
    ]
  },
  "environments": [
    {
      "name": "dev",
      "config": {
        "plugins": {
          "extractors": [
            {
              "name": "example",
              "env": {}
            }
          ]
        }
      }
    }
  ]
}

Before the changes to the schema:

After the changes to the schema (plus removing the ID in the validator to prevent a conflict):

Closes #7037

[netlify]

✅ Deploy Preview for meltano canceled.

Name	Link
🔨 Latest commit	e09ea5f
🔍 Latest deploy log	https://app.netlify.com/sites/meltano/deploys/639357af39a05d0009c226e6

[codecov]

Codecov Report

Merging #7064 (e09ea5f) into main (48a64d3) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #7064      +/-   ##
==========================================
+ Coverage   88.64%   88.65%   +0.01%     
==========================================
  Files         292      292              
  Lines       21304    21305       +1     
  Branches     2106     1686     -420     
==========================================
+ Hits        18884    18888       +4     
+ Misses       2063     2060       -3     
  Partials      357      357              

Impacted Files	Coverage Δ
tests/meltano/cli/test_run.py	98.38% <100.00%> (+<0.01%)	⬆️
src/meltano/cli/utils.py	83.80% <0.00%> (+1.05%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more