Support Month in Duration Literal for Group by Operation

Issue Summary:
In InfluxQL, there's a lack of a built-in duration specifier for months, which makes it challenging to accurately group data into monthly intervals. Currently, users have to resort to approximations using days, which can lead to inaccuracies, especially when dealing with varying month lengths and leap years.

Proposal:
I propose adding a month duration literal to InfluxQL to provide users with a more intuitive and precise way to specify monthly intervals in queries. This enhancement would streamline data analysis and reporting workflows, making it easier for users to work with temporal data at the monthly level.

Suggested Syntax:
A suggested syntax for the month duration literal could be 1mo, following the conventions used in other date-time libraries and query languages.

The issue names no files, tests, or entry points. Start by locating InfluxQL duration-literal parsing and GROUP BY handling, then determine how the proposed 1mo syntax should be represented and tested. Done means monthly grouping accepts the month literal without relying on day approximations.

Written by the indexing model from the issue text.