Add mutual SSL support & improve test coverage

[dilanSachi]

Purpose

$subject
Fixes ballerina-platform/ballerina-library#4636

Examples

Checklist

• Linked to an issue
• Updated the changelog
• Added tests
• Updated the spec
• Checked native-image compatibility

[codecov]

Codecov Report

Merging #19 (dbc80d0) into main (7d689b9) will increase coverage by 38.91%.
The diff coverage is 86.42%.

@@              Coverage Diff              @@
##               main      #19       +/-   ##
=============================================
+ Coverage     45.92%   84.84%   +38.91%     
- Complexity       21       60       +39     
=============================================
  Files            12       14        +2     
  Lines           307      409      +102     
  Branches         42       62       +20     
=============================================
+ Hits            141      347      +206     
+ Misses          150       54       -96     
+ Partials         16        8        -8     

Impacted Files	Coverage Δ
.../java/io/xlibb/mqtt/listener/MqttCallbackImpl.java	77.77% <72.72%> (+41.88%)	⬆️
...e/src/main/java/io/xlibb/mqtt/utils/MqttUtils.java	84.48% <82.79%> (+30.91%)	⬆️
ballerina/listener.bal	96.87% <94.11%> (+21.87%)	⬆️
...c/main/java/io/xlibb/mqtt/utils/MqttConstants.java	96.29% <96.29%> (ø)
ballerina/errors.bal	100.00% <100.00%> (ø)
ballerina/types.bal	92.85% <100.00%> (+11.03%)	⬆️
.../main/java/io/xlibb/mqtt/caller/CallerActions.java	66.66% <100.00%> (+66.66%)	⬆️
.../main/java/io/xlibb/mqtt/client/ClientActions.java	83.72% <100.00%> (+46.51%)	⬆️
...n/java/io/xlibb/mqtt/listener/ListenerActions.java	81.63% <100.00%> (+33.90%)	⬆️

... and 2 files with indirect coverage changes

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