add expression functions to create CRS from string definition and to get CRS authid (refs #58313)

[alexbruy]

Description

Processing batch mode allows to defined output name using expressions and all algorithm parameters are available as values in this context. However, CRS parameters when used in expression return nothing. This happens because when algorithm scope is populated we create variables using the values obtained from parametersForRow() method and these values are just uses as-is

QGIS/python/plugins/processing/gui/BatchPanel.py

Lines 956 to 961 in 6b4ffb3

	for param in self.alg.parameterDefinitions():
	if param.isDestination():
	continue
	col = self.parameter_to_column[param.name()]
	wrapper = self.wrappers[row][col]
	parameters[param.name()] = wrapper.parameterValue()

In case of CRS parameter, variable will be populated with the instance of QgsCoordinateReferenceSystem. Expression engine does not have support for variants containing QgsCoordinateReferenceSystem and evaluating such variable returns nothing as reported in #58313.

This PR adds two new expression functions:

• make_crs(definition) to create a QgsCoordinateReferenceSystem from a definition
• crs_authid(crs) to get a string with the CRS authid

Refs #58313.

[alexbruy]

@nyalldawson what do you think?

Another approach would be to evaluate parameter value for CRS parameter in the BatchPanel and use authid when settings corresponding variables.

[github-actions]

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[nyalldawson]

Can you also add the expression function json documentation for these?

[nyalldawson]

@nirvn also keen to hear your thoughts