fix(template): allow partially resolved vars in arithmetic expressions

[vvagaytsev]

What this PR does / why we need it:

This PR allows using partially resolved variables in arithmetic operations.

Which issue(s) this PR fixes:

Fixes #6201

Special notes for your reviewer:

If we omit the explicit double-quotes in the echo command args in the example in #6201, i.e. replace

spec:
  command: [echo, '"${var.prefix.suffix * 4}"']

with

spec:
  command: [echo, '${var.prefix.suffix * 4}']

we'll get a schema validation error:

Error validating spec for Run type=exec name=my-test:

At path spec.command.1: Expected string, received number

That is not related to the template string resolution and should be fixed in a separate PR. A possible solution is to stringify all numbers in spec.command's args implicitly.

[vvagaytsev]

@twelvemo @10ko it's still a draft, sorry for the early review request.

[vvagaytsev]

@twelvemo @10ko it's ready for review now :)

[10ko]

Looks good!