feat(helm): Add Files.AsSecrets, Files.AsConfig, path functions

[andrewstuart]

This PR adds a number of useful utilities for working with files and file paths.

• Several of the StdLib path functions have been exposed to templates for working with file paths
• The Files type learned to serialize itself to valid YAML maps for both config maps and secrets via Files.AsConfig() and Files.AsSecrets()

A few possible edits I was unsure on:

• AsConfig() and AsSecrets() could instead return maps intended to be passed to toYaml or toJson(does that exist?) to support either use case. I chose YAML because I think that's probably the most commonly used language, but I know there are people out there using JSON.
• This could be more composable. For example, {{ Files.Glob "secrets/*" | secretMap | flattenDir | toYaml }}.
• I'm not attached to the method names at all. There may be more semantic/evocative names for what these methods do. I just didn't want to overdo it with ToKubeConfigMapYAMLKeys() or some such nonsense. 😄

[k8s-reviewable]

This change is 

[technosophos]

I guess my preference would be to have these path functions rolled into Sprig. We try to only put "special" functions here. If you put them in Sprig, I'll roll another Sprig release ASAP, too.

[technosophos]

Other than discussing the possibility of moving the path functions to Sprig, I am on board with doing this.

[technosophos]

I don't suppose I can coerce you into also doing this one too, could I? ;-) #1548

[technosophos]

Manually tested