add grammar support for @modifier.

[yu-shipit]

Pull Request checklist

• The commit(s) message(s) follows the contribution guidelines ?
• Tests for the changes have been added (for bug fixes / features) ?
• Docs have been added / updated (for bug fixes / features) ?

Current behavior :
Filodb does not handle @modifier.

New behavior :
filodb will be able to parse @modifier.
will support query parsing like

foo @timestamp,  foo @start(),  foo @end()
foo[1m] @timestamp,  foo[1m] @start(),  foo[1m] @end() 
rate(foo[1m] @timestamp),  rate(foo[1m] @start()),  rate((foo[1m] @end())

foo @timestamp offset 1d, foo offset 1d @timestamp
foo[1m] @timestamp offset 1d, foo[1m] offset 1d @timestamp
rate(foo[1m] @timestamp offset 1d), rate(foo[1m] offset 1d @timestamp)

Note that the an at modifier can coexist with an offset modifier irrespective of order.

Breaking changes may include:

• Changes to query parsing / PromQL parsing

[alextheimer]

LGTM, just one small update requested in the comments-- please feel free to dismiss this review after that's updated.

[alextheimer]

Approved for the sake of forward progress -- please feel free to merge after the other comments are addressed.

[alextheimer]

Thanks for the additional tests @yu-shipit 🙏