fix: Add support for .net 6 runtime (closes #614)

[maciej-bielecki]

What did you implement:

Added .net6 runtime version to configs. Without it we were not able to use this runtime in serverless.yml. Closes #614

How did you implement it:

I've added DOTNET60 to enum Runtime, other necessary structures and register commands so that it can be build.

How can we verify it:

Use runtime: dotnet6.0 in serverless.yml file. Use project targeting net6.0, also azure functions v4 can be used:

Examples:

• serverless.yml - provider part

  ...
  provider:
  name: azure
  region: West Europe
  functionApp:
  extensionVersion: '~4'
  runtime: dotnet6.0
  ...

• .csproj - target version for net6.0

  <Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
  <TargetFramework>net6.0</TargetFramework>
  <AzureFunctionsVersion>v4</AzureFunctionsVersion>
  ...

  Todos:

Note: Run npm run test:ci to run all validation checks on proposed changes

• [x] Ensure there are no lint errors.
  Validate via npm run lint
  Note: Some reported issues can be automatically fixed by running npm run lint:fix
• [x] Write tests and confirm existing functionality is not broken.
  Validate via npm test
• [ ] Write documentation
• [x] Provide verification config / commands / resources
• [x] Enable "Allow edits from maintainers" for this PR
• [x] Update the messages below

Is this ready for review?: YES Is it a breaking change?: NO

[alexanderwink]

Any plans on getting this PR merged?