pkg/aflow: add ability to generate several candidate replies for LLM agents

[dvyukov]

Add LLMAgent.Candidates parameter.
If set to a value N>1, then the agent is invoked N times,
and all outputs become slices.

The results can be later aggregated by another agent,
as shown in the test.

[dvyukov]

cc @FlorentRevest @sirdarckcat don't know how useful this is for your experiments, but this allows to create workflows along the lines of:

&LLMAgent{
	Name:        "patch-generator",
	Reply:       "PatchDiffCandidate",
	Candidates:  5,
	Instruction: "Generate bug fix patch",
	Prompt:      "Crash: {{.Crash}}",
},
&LLMAgent{
	Name:        "aggregator",
	Reply:       "PatchDiff",
	Instruction: "Pick the best fix",
	Prompt: `
Crash:
{{.Crash}}

{{range $diff := .PatchDiffCandidate}}
Proposed fix:
{{$diff}
{{end}}
`,
},

[sirdarckcat]

Nice! I'll try to help adding tools during the hackathon