Add a conditional formatting rule (boolean highlight or color gradient) to a range in a Google Sheet.

Documented steps

1. POST https://sheets.googleapis.com/v4/spreadsheets/{spreadsheetId}:batchUpdate using OAuth scope https://www.googleapis.com/auth/spreadsheets.
2. Add requests[0].addConditionalFormatRule.rule.ranges = [GridRange, ...] — one or more zero-based, half-open ranges the rule applies to (required).
3. For a highlight rule, set rule.booleanRule.condition (e.g. {type:"NUMBER_LESS_THAN_EQ", values:[{userEnteredValue:"10"}]}) and rule.booleanRule.format (e.g. {backgroundColor:{red:0.8, green:0.2}}).
4. For a color-scale rule, set rule.gradientRule with minpoint and maxpoint (each {color, type: MIN|MAX|NUMBER|PERCENT|PERCENTILE}) and an optional midpoint, instead of booleanRule — only one of booleanRule/gradientRule may be set per ConditionalFormatRule.
5. Set requests[0].addConditionalFormatRule.index to the zero-based position at which to insert the rule among the sheet's existing conditional format rules.
6. Docs: https://developers.google.com/workspace/sheets/api/reference/rest/v4/spreadsheets/request | https://developers.google.com/workspace/sheets/api/samples/conditional-formatting | https://developers.google.com/workspace/sheets/api/limits