[github] support expressions

[jimsheldon]

An expression can be any combination of literal values, references to a context, or functions. You can combine literals, context references, and functions using operators.

We should have a way of converting GitHub expressions (${{ ... }})into Harness expressions (<+ ... >).

See

• https://docs.github.com/en/actions/learn-github-actions/expressions
• https://docs.github.com/en/actions/learn-github-actions/contexts
• https://developer.harness.io/docs/platform/references/runtime-inputs/#expressions

[jimsheldon]

There are expressions that may not be supported in Harness CI, such as hashFiles.

[jimsheldon]

I think as part of this, we need a package within go-convert that lets us define things like <+trigger.prNumber> and <+trigger.payload.pull_request.body> in one place.