dev/internal: create team package

Created by: bobheadxi

Extracts teammate data resolving out of buildchecker into a separate package, and extends it significantly such that it can also replace the teammate querying in sg teammate.

❯ go run . teammate handbook jh  
Opening handbook link for JH Chabran: https://handbook.sourcegraph.com/team#jh-chabran

❯ go run . teammate time jh    
JH Chabran's current time is 13 Jan 22 02:11 CET (1h from your local time)

For https://github.com/sourcegraph/sourcegraph/issues/25454 - this will give us an API we can use for generating direct message notifications in the pipeline.

Package usage:

import "github.com/sourcegraph/sourcegraph/dev/internal/team"

func main() {
	// ...

	// Neither a GitHub client nor a Slack client is required, but each enables more ways
	// to query for users and/or get additional metadata about a user.
	teammates := team.NewTeammateResolver(githubClient, slackClient)
	tm, _ := teammates.ResolveByName(ctx, "Robert")
	println(tm.SlackID)
	println(tm.HandbookLink)
	println(tm.Role)
	// etc.
}