Add post-render strategy support and conditional defaults

[vocarista]

Fixes #1469

⚠️ Breaking change: From Flux 2.8.0 to 2.8.5 the default was combined (Helm 4 breaking change), then in Flux 2.8.6 we fixed the regression (changed the behavior to nohooks). In Flux 2.9 (through this PR) we are changing this default back to combined for honoring the feature gate UseHelm3Defaults, which defaults to the Helm 4 defaults when the gate is false, and to the Helm 3 defaults when the gate is true. We are sustaining our decision to keep aligned with Helm's defaults throughout Flux's and Helm's lives. This commitment implies shipping breaking changes whenever Helm changes default functionality in major versions.

The behavior can also be pinned to a specific value directly in the HelmRelease object through the field introduced in this PR.

Adds support for Helm's post-render strategies (nohooks, combined, separate) being introduced in Helm 4.2. The new spec.postRenderStrategy field allows users to configure how hooks are sent to post-renderers.

Changes:

• Add PostRenderStrategy enum with values: nohooks, combined, separate
• Add spec.postRenderStrategy field to HelmReleaseSpec
• Add toHelmPostRenderStrategy() mapping function in action package
• Update install and upgrade actions to use the configured strategy
• Add unit tests for default behaviour and all strategy values

Default behavior:

• Defaults to combined to align with upstream Helm
• Respects UseHelm3Defaults feature gate by switching to nohooks when enabled

[matheuscscp]

Just a nit, otherwise LGTM! 🚀

[matheuscscp]

@vocarista Sorry, I forgot something very important, please add the field docs to docs/spec/v2/helmreleases.md 🙏

[matheuscscp]

Then please squash the commits into one and force-push 🙏

[vocarista]

Squashed 👾

[matheuscscp]

@vocarista Can you please open a PR here too? https://github.com/fluxcd/website/blob/main/content/en/flux/components/helm/options.md?plain=1#L67

Then a second PR here (highlighting the breaking change):
https://github.com/fluxcd/website/blob/main/content/en/roadmap.md?plain=1#L41

They have to be two separate PRs because the second one will go live immediately

[vocarista]

@matheuscscp I've opened the PRs on fluxcd/website

[matheuscscp]

LGTM! 🚀

@vocarista Thanks very much!