Add template for client tools auto-update download url

[vapopov]

In this PR introduced templating feature for client tools download url as well as ability to customize it by environment variable

Default template:

https://cdn.teleport.dev/
{{- if eq .OS "darwin" }}
{{- .Package }}{{ if and .Enterprise (eq .Package "teleport") }}-ent{{ end }}-{{ .Version }}.pkg
{{- else if eq .OS "windows" }}
{{- .Package }}-v{{ .Version }}-windows-amd64-bin.zip
{{- else }}
{{- .Package }}{{ if .Enterprise }}-ent{{ end }}-v{{ .Version }}-{{ .OS }}-{{ .Arch }}{{ if .FIPS }}-fips{{ end }}-bin.tar.gz
{{- end }}`

Might be overridden by TELEPORT_URL_TEMPLATE environment value

Changelog: Added ability to define download URI for client tools managed updates for custom CDNs