Greater flexibility in custom filter functions

[bkoelman]

This PR enables using a (built-in or custom) function as the first parameter of the any/contains/startsWith/endsWith query string functions. For example, the following filters are now allowed (where toUpperCase is a custom function):

?filter=any(toUpperCase(title),'A','B','C')
?filter=any(lessThan(votes,10),'true')
?filter=contains(toUpperCase(title),'EXAMPLE')
?filter=startsWith(toUpperCase(title),'EXAMPLE')
?filter=endsWith(toUpperCase(title),'EXAMPLE')

Additionally, this PR adds an example that defines a custom filter function, backed by a user-defined SQL function.
For example, in a query string such as:

?filter=equals(decrypt(password),'secret')

The decrypt custom filter function is substituted by the decrypt_column_value() user-defined SQL function in the generated WHERE clause.

CREATE OR REPLACE FUNCTION decrypt_column_value(value text)
    RETURNS text
RETURN your_crypto_logic_here;

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.21%. Comparing base (1e33bd4) to head (22488f8).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...ore/Queries/Expressions/QueryExpressionRewriter.cs	66.66%	0 Missing and 2 partials ⚠️
...ApiDotNetCore/Queries/Expressions/AnyExpression.cs	85.71%	0 Missing and 1 partial ⚠️
...NetCore/Queries/Expressions/MatchTextExpression.cs	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1881   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         437      437           
  Lines       14866    14880   +14     
  Branches     2451     2454    +3     
=======================================
+ Hits        13708    13722   +14     
  Misses        713      713           
  Partials      445      445           

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