readonly support for the parameters of remote methods

[ayeshLK]

Purpose

$subject

Related to #2604

Examples

With this change, developers could use readonly parameters for remote methods in websubhub:Service declaration:

service /websubhub on hubListener {
    isolated remote function onRegisterTopic(readonly & websubhub:TopicRegistration msg) returns websubhub:TopicRegistrationSuccess|websubhub:TopicRegistrationError {
        // implement logic here
    }

    // implement other remote methods
}

readonly can only be applied to records declared in WebSubHub package. Hence, following will be invalid:

service /websubhub on hubListener {
    isolated remote function onRegisterTopic(readonly & websubhub:TopicRegistration msg, readonly & http:Headers headers) returns websubhub:TopicRegistrationSuccess|websubhub:TopicRegistrationError {
        // implement logic here
    }

    // implement other remote methods
}

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #424 (b250ffe) into main (0f2b6d5) will increase coverage by 2.16%.
The diff coverage is 94.73%.

@@             Coverage Diff              @@
##               main     #424      +/-   ##
============================================
+ Coverage     85.90%   88.06%   +2.16%     
- Complexity       22       37      +15     
============================================
  Files            15       15              
  Lines           759      796      +37     
  Branches        449      461      +12     
============================================
+ Hits            652      701      +49     
+ Misses          105       91      -14     
- Partials          2        4       +2     

Impacted Files	Coverage Δ
...stdlib/websubhub/NativeHttpToWebsubhubAdaptor.java	94.93% <94.73%> (+6.84%)	⬆️
ballerina/commons.bal	91.80% <0.00%> (+3.27%)	⬆️
ballerina/subscription.bal	91.73% <0.00%> (+6.61%)	⬆️
ballerina/natives.bal	100.00% <0.00%> (+7.69%)	⬆️

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