add basic plumbing for provider startup checks

[jastang]

Description of your changes

This PR proposes a simple approach to enable custom startup checks on the provider. As a concrete example, users may run their workloads in highly regulated environments where the provider should not start if the environment is not properly configured.

This implements the default path, which is a no-op. It uses a combination of an init() hook and go build tags to:

• minimize code branching (build-time isolation)
• ensure critical checks run before anything else (fail fast)
• not require access to additional flags, env, or setup

Users can implement an array of preflight checks as separate files in this repository or a fork, and supply GO_TAGS=<..> to build an image with a specific one.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review.

How has this code been tested

locally built xpkg installation.

[jeanduplessis]

LGTM