Add explain slash command

What does this MR do and why?

Related to #431575 (closed)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Example below:

1. Feature.enable(:slash_commands)
2. In one browser tab open Duo Chat UI, in second tab open graphql-explorer
3. Send the following request from graphql explorer and check the result in Duo Chat UI:

mutation {
  aiAction(input: {chat: {content: "/explain", currentFile: {
    fileName: "test.rb",
    selectedText: "
def extract(content)
  lines = content.to_s.lines
  comment_block = []
  trimmed_lines = 0
end
",
  }}}) {
    clientMutationId
    errors
  }
}

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.