feat: updates logger interface for Functions

[chaitanyakolluru]

functions

Description of your changes

This change sets up FnLogger with Error() method and its implementation to allow for functions to log error instances along with adding them to RunFunctionResponse's Results array.

Fixes #143

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

Manually tested against a local function that uses FnLogger in place of crossplane-runtime's logger as shown below:

type Function struct {
	fnv1beta1.UnimplementedFunctionRunnerServiceServer

	log    logging.FnLogger
}

[chaitanyakolluru]

Closing pull request as using Info for logging fulfills our use-case.