refactor: add apply_window_if_present and get_window_order_by methods

[chelsea-lin]

Fixes internal issue 430350912 🦕

[chelsea-lin]

This change is one of break-down changes for #1889

[sycai]

For time window rolling like this, we need to wrap the rolling column with unix_micros()

The result should look like this:

... OVER (ORDER BY UNIX_MICROS(`col1`) ASC ...) 

Could you update the compiler code to make it happen?

[sycai]

I have an example code for you to quickly verify whether the generated SQL is valid E2E:

import bigframes.pandas as bpd
import pandas as pd

df = bpd.read_gbq("bigframes-dev.sycai.test_rolling")

df.set_index('ts_col').sort_index().rolling(window=pd.Timedelta('3s')).min().sql

[chelsea-lin]

This pull request (PR) focuses on implementing just two utility functions for windows, while the full windows compilation is drafted in PR #1889. With the full implementation, the e2e codes can generate the right golden SQL (see here). IIUC, the ops.UnixMicros operator will response for the SQL UNIX_MICROS generation. Let me know if you have further questions.

[sycai]

Got it! Thanks for the explanation