fix(stdlib): Return string from base64_decode

[jplitza]

Brief description of Pull Request

This changes the return type of the encoding.from_base64() and encoding.from_URLbase64() functions in the stdlib from []byte to string.

Pull Request Details

Previously, encoding.from_base64() returned []byte, which could not be cast to secret and hence could not be used in places where a secret was expected. The error was:

encoding.from_base64(...) should be capsule, got array

Converting bytes to a string needs to assume an encoding. Go should use UTF-8 here. This conversion happened implicitly before the change whenever the result was used as string.

Issue(s) fixed by this Pull Request

Fixes #5450

Notes to the Reviewer

PR Checklist

• Documentation added
• Tests updated
• Config converters updated
• This pull request was substantially generated with AI assistance (see the GenAI policy)

[github-actions]

Signed commits report

All 1 commit between main and patch-1 have verified signatures. ✅

[jplitza]

I uppercased the PR title, but I fail to see how this failing test has anything to do with my changes or how to fix it:

--- FAIL: TestLokiAzureEventHubs (2.33s)
    loki_azure_event_hubs_test.go:39: 
        	Error Trace:	/home/runner/work/alloy/alloy/integration-tests/docker/tests/loki-azure-event-hubs/loki_azure_event_hubs_test.go:39
        	Error:      	Received unexpected error:
        	            	failed to send message to topic "test1": kafka server: Tried to send a message to a replica that is not the leader for some partition. Your metadata is out of date
        	Test:       	TestLokiAzureEventHubs