prompts: FewShotPrompt implementation

[AMK9978]

Description

Added FewShotPrompt. Fixes #196.
Python source

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• Describes the source of new concepts.
• References existing implementations as appropriate.
• Contains test coverage for new functions.
• Passes all golangci-lint checks.

[tmc]

if we're putting multiple fields in here then the options argument to New shouldn't be variadic

[AMK9978]

I wanted to let developers not provide any option instead of passing an empty CallOption instance. I guess we have the same with the llms.options.

[FluffyKebab]

I don't think having options to the constructor is necessary for this.

[AMK9978]

What about default values?

		ExampleSeparator: "\n\n",
		TemplateFormat:   TemplateFormatGoTemplate,
		ValidateTemplate: true,

these are their default values. I can set their type as a pointer to let developers set nil and I set their default. I want them to be able to use default values without explicitly setting them. Thoughts?

[AMK9978]

@FluffyKebab
Could you take a look at this PR? It's essential for another PR which is ConstitutionalChain.

[FluffyKebab]

I don't think having options to the constructor is necessary for this.

[AMK9978]

@FluffyKebab @tmc
I updated the code and removed FewShotCallOptions. I will appreciate it if you take another look at it.